Ok here is the vital stats. Its a 1998 Nissan Sentra GXE Automatic with about 91,000 miles on it. At first I thought it was a bad battery. When I had it tested the battery AND the alternator were bad... Replaced both of them. Now, at random, the car will not start. It doesnt mater if it is warm or cold, what time of day it is, raining or dry, if I drove it for 2 minutes or 40 before turning it off... it truely is random.

When it doesnt start a few things happen and dont happen.

- everything works with the key in. Headlights are bright, radio works fine, wipers, horn, windows and locks all work fine.

- there is a small click from a relay in the fuse box under the drivers dash and you can hear the fuel pump run for about 2 seconds priming the fuel system... but that is it. No selinoid click, no starter spinning... it is as if the battery were totally dead.

Except, the battery is holding a steady 12.56-12.55 volts with the car off. When I try to start it, battery voltage drops to about 12.25-12.3 then returns right back to 12.55 when you let the key go. (when the car is running the alternator is pumping out right around 14 volts... in that 14-14.5 range)

I have also cleaned the battery ground (where it connects to the trans case) and sanded the cable terminals on both power and ground side of the battery.

- All fuses have been checked (dont know how to test relays as there seems to be a different way to check the 1000 different relay types)

Now here is where it gets super weird. If the car starts, it is as if the car is perfect. It has been to 2 mechanics and both have said the car is fine (but neither could get the car to not start while they had it)

- If it wont start, a jump from a running car will start it. A boost from a booster pack will not.

- Resetting the ECU by taking the battery out for half an hour does nothing.

- Once jumped and running, it will start again for a few hours, or a few days, but it WILL fail to start again eventually.

Even when being jumped the starter will crank slow as if the battery were dead. It will make 2 or 3 full (insert cranking noise here) and then 1 or 2 half hearted, almost dead battery type cranks (it gets very slow and quiet) then on the last half hearted crank, just when a battery would be completly drained, it "catches" and the engine starts.

- If it wont start and I just leave it alone, it will start again in a few hours to a few days. (I actually found this out one day when it died and I walked to work... I came home and my car was on the other side of the street. Apparently about 4 hours after I left, my GF hopped in, started it, and drove to the store... as if nothing was wrong with it)

I cant check power to the starter because it is in a major PITA spot, and I can not get in there with a multimeter AND crank at the same time. If the car starts my entire body is pretty much touching moving parts that will shred me and I dont have long enough test leads to hook it up, get out, and crank the car.

What do you guys think? I have had people tell me everything from "oh its your starter" to "it could be your ignition switch" to "sounds like it is a bad ground somewhere"

Also does anyone know the location of all the grounds on a 98 Sentra? I am tempted to clean or replace all of the ground wires in the entire car

The shops you took it to didn't test the starter? It certainly sounds like it could be the issue but without being there, going to be very difficult to figure out.

For the grounds, need to know if it's the SR20 or the GA18.

It sounds to me, like you have an issue with the grounding of the vehicle. Get a multimeter and check the voltage at the battery and write it down. Then check the voltage between the positive pole of the battery and bear metal at the engine/transmission. Both should be very close if not you have a ground fault. Also check the voltage at the 30 Amp Fusible link and body ground. Check the voltage between the 75 Amp Fusible link and ground, both of these should be at battery voltage. A loose /dirty/rusty connector can cause this type of fault as well.
What I would do if I were you is to get a new battery lead for the negative/ground. Disconnect the old one and connect the new one to the negative pole (battery) and the other end to a nice clean bolt on the transmission as close to the starter as possible.

Once you are not getting blown fuses or blown fusible links then it has to be a grounding problem or loose connection somewhere. Start with the negative lead first.

I forgot I created this topic. Just so everyone knows, or if someone else has this problem again, I will let you know what the final solution was.

It ended up being an internal wiring issue with the starter. Hence why it was giving all the symptoms of an electrical problem (Because IT WAS) but the electrical problem was inside the starter. It finally got worse and worse to the point where jumping the car wouldn't start it anymore, and it completely died. (its so much easier to fix things that always don't work, instead of sometimes works, sometimes doesn't)

New starter and we were on our way again. 4 months without a single starting issue now