1990 Nissan 300ZX troubleshooting... please HELP!!!

[Nissan300Zfor2]

Well... here I go (again). I have a 1990 Nissan 300ZX Non-Turbo and had some trouble with the engine getting hot, someone had told me to change the Thermostat and the Fan Clutch. I put the front of the vehicle on stands to get to the lower Radiator Hose and actually left the vehicle on the stands for 3-4 hours (I never got to changing the thermostat or the Fan Clutch). When I took the Z off the stands and started the vehicle, it was shaking badly and the sound out of the muffler didn't sound as solid as it did before. I didn't diagnose it, but it seemed that one of the Cylinders wasn't working properly. I took the Coils out and the spark-plugs, the plugs were changed less than a year ago, but was showing some black built-up on the round part on the bottom. Put them back in and the car is still shaking. It seems that I am losing power as well. I am totally lost on what it could be. Is there a way to run a diagnose check? Any other suggestions... or did anyone have a similar problem like this? Like I said, before I put the car on the stands, I did NOT have this problem with the car shaking. PLEASE HELP ME!!!

[freakonaleash1187]

Try running an ECU diagnosis, that may tell you what the problem is:

zerothread/249406

[evildky]

did the car get really hot right before you put it on stands? could be a HG