I have a 96 z34 and it has 123,000 miles on it. Today it would not go over 3000rpms and when it got to 3000 there was a knocking noise that sounded like it was below the pedals. I just changed all the plugs in it. If you guys have any ideas on what it could be, I would greatly appreciate it.

 

could be a clogged cat or tranny solenoid

 

One troubleshooting principle that I always try and adhere to is look at the last thing I did - in this case plug change. Any chance of crossed wires? Was it running fine before the change? You don't have a rev limiter do you? I'd recheck the wiring and make sure the firing order is correct...and that all the plugs are fine and gapped correctly....

 

This in gear or in park/neutral? as there is a rev limiter, but its closer to 4k.