Slips to neutral at 45mph: transmission or other?

[dogslovetrucks]

Hello folks-- I'm new to the Nissan world. Just bought a '92 Hardbody 4cyl auto (settled on auto) only 125k a couple months ago from a single-owner old lady who was selling. Given my respect for Nissans though I haven't ever owned one, I bought it anticipating it might last me 10+ years given the low mileage. This is a comprehensive explanation of some transmission problems that I'm trying to figure out.

As background, this was my first 4cyl and I attributed that to the reason this truck had pretty poor pickup. This is all hindsight speaking now, but it also sounded and still sounds like it is struggling to suck air at almost every speed and acceleration from 45 to 60 has always been pathetic. It also had a rough idle when I got it-- when idling in gear it would often flutter between 300-600rpm. I changed the plugs, wires, and air filter. When nothing changed, I popped in a new PCV valve as I changed the oil. When nothing changed, I put in a new idle speed solenoid. When not much changed, I wanted to do more and didn't have time to disassemble and thoroughly clean the throttle body so I just sprayed a bunch of carburetor cleaner in the throttle body, even shooting short bursts as the truck idled into whatever the airflow is called that passes by the idle speed solenoid when the throttle is closed. (I'm not an expert on these matters, and actually that spraying may have been a problem which is why I'm 'fessing up to it.) This surface-level cleaning really helped smooth out almost all the rough idle.

A week after that spraying and during the coldest stretch of January (which here in NC means some snow/ice and 20s most of the day), I was driving home from work and my transmission started slipping at 45mph. It shifts fine up and down but once I hit 45mph with overdrive on, it is as if the transmission slips into neutral: rpms go from 2k to 3k with no forward acceleration. It seems I can kinda push a few more mphs out of it, and especially if I turn off the overdrive-- it goes up to 50+ until I just don't want to spin that many rpms without knowing what is wrong with this. Also-- since I have had the truck, I have always noticed that the spedometer seemed off at least in the <50 speeds. I felt like I was doing 35 though it would register at 25. I noticed the other day when I drove it in its now hobbled form that the spedometer is completely screwed up: it gets stuck at random places, seems to be low most often, and takes its time falling back to 0.

I've read all sorts of posts about transmission issues. I fear having to rebuild my transmission-- but this problem really seems electrical. I hope it is electrical. I have a few theories. To start with, and please correct me if I'm wrong, but the son of the previous owner told me this truck is too old to be able to pull codes on it. So my ideas are in my current order of suspicion:

1) Vehicle speed sensor and/or EGR valve. I've heard of a bad VSS causing my exact "transmission" problem in other Nissan cars so I have hope that this is it. I've also heard that a VSS is often not the culprit but a bad EGR screws up its reading. Should I start with replacing the EGR valve? Can I check it somehow?
2) Throttle position sensor. I checked my wires out of my TPS and oddly found an unconnected uninsulated stranded wire floating around within the taped up TPS wiring. I have no clue what that is there for but unconnected wires are pretty odd. I had already suspected TPS problems could be the cause and finding that wire makes me wonder if its operating properly. Anyway to check the TPS?
3) Fuel filter change/transmission fluid flush. Being a used vehicle, these steps I feel like can't hurt and I should probably just do them anyway since I don't know the last time they were done.
4) Transmission solenoid malfunction. My understanding after looking at the Nissan Transmission Manual is that these are internal. Mechanics have told me that in order to get to them will mean that I'll have to have the transmission rebuilt after cracking it open. I kinda want to fact-check that and ask if this is a possible explanation.
5) Vacuum tube problem. I did notice that after I sprayed the throttle body down with cleaner, I forgot to replace a vacuum tube that was coming off the bottom of the fuel filter holder. Oops. I hope having that off for a week would not have screwed up anything. That was just going to some sensor that is on the downwind side of the filter.
6) Mass airflow sensor. It seems like the MAF often makes lots of troubles, but I don't know how a bad MAF could lead to this neutral at 45mph problem. Nonetheless, given the rough idle problems and general air flow concerns, the MAF is always on my list.
7) Catalytic converter. This is kinda on the EGR follow-up. Seems like some folks have reported that a bad/clogged catalytic converter can cause EGR problems which can cause performance problems. Not sure if it can lead all the way back to this neutral at 45mph thing, but hey, it's on the watch list.

ANY thoughts would be MUCH appreciated! Even just a command of "take it to a shop, they can pull some codes for you" would give me some direction. I've been feeling paralyzed and am on a budget-- I bought a 21 year old truck because that's what I could afford. A $1600 transmission rebuild is not friendly. Even buying a $100 EGR valve is something I want to do only if it makes sense. Thanks!

[Rev_D21]

Welcome to NICO! It sounds like your speedo cable has bit the dust at the very least. It's a dealer-only item unless you can find a used one but then you have to be sure it's the right one for your truck. This is why the needle is stopping in various areas along with general weirdness. As for the engine being a dog I would check the timing. It sounds like the distributor is off by a tooth which causes poor idle and very doggish performance. Set the #1 piston to TDC then remove the distributor cap. Check the position of the rotor. It should be at 4 o'clock with the number one piston at TDC. If it isn't then you know the distributor and oil pump are on the wrong tooth. As for the transmission slipping into neutral, sounds like the torque converter or the clutches inside the transmission itself are on the way out. I have a 1991 auto and lost all forward gears last Fall in the same manor yours is about to. Since the rest of the truck is in decent shape I am opting to swap to a 5 speed transmission.