runs rough and no power uphill

[JCMITO]

All of a sudden, my 91 240sx has no power uphill and runs a bit rough at idle. I changed the spark plugs, clean the cap and rotor which I had replaced about 6,000 miles ago. all looks fine and there is no damage. I checked the timing and it's at 20 above TDC. Replaced the ignition coil, air filter, fuel filter and I'm still having the same issue. I don't get an engine light. at idle is at 700rpm, when shifting to drive, the rpm drops to about 600 and the car shakes a bit more as if it wants to stall, but it never does. I took one spark plug out at a time and the car runs rougher, so all cylinders seem to be running. I intend to do a compression test tomorrow. I'm running out of ideas on what can the problem be. Can anyone give me some ideas on what I can check next? Thanks a lot.

[MaximA32]

Check all the vacuum lines. Have you ever cleaned the IAC/V?

[JCMITO]

I dont' know how to clean it. At any rate, after checking compression (200-180-17- & 160) and the timining 20 above TDC, I gave up. I checked hoses in addition to above. I ended up taking to the mechanic since I ran of idean on what to do. It turned out to be one of the injectors. He charged me $200; i guess each injector is about $89. I thought about doing all, but he said it wasn't necessary. I hope this helps if someone looks for information.

[MaximA32]

I would still clean it. You can see the tutorial here: iacv-aac-cleaning-and-testing-tutorial-t251133.html

[JCMITO]

That's great. Thanks.
The car started having the same problem a few days later; I have a feeling that maybe one of the other injectors failed. I thought the connection of the one changed by the mechanic may be loose; no such luck. I read some where that it would have been a good idea to change them all. Is this what it is recommended? I need to check out how to change them to see if I can do it.