I have a 1998 Z24 with 5 speed manual. Over the past several months I have been experiencing a engine stumble (mis-fire?) under medium to hard acceleration. If I accelerate lightly, the problem goes away. Recently I noticed that this problem does not start until after the car warms up. I live in Wisconsin so now the problem begins after several minutes of driving. No noticeable change in MPG, oil consumption, smoke, idle smoothness, etc.

Things I have tried and replaced recently:
- EGR valve (replaced a year ago and recently tried unplugging which did not solve problem)
- TPS sensor.Triednew unitand did not solve problem.
- Replaced both coils, ignition housing (no plug wires with this motor) and spark plugs about 4 months ago.

I am at a loss. It has to be related to either ignition or fuel. Does this motor have a cold start injector? Or some type of system that alters fuel delivery for a cold engine?
I could go back and try replacing the spark plugs and housing body but that does not seem likely since the problem is not present when cold.

Maybe related, maybe not but in the summer months, there is a idle problem shortly after start up. Within a minute of starting, the car will almost stall itself. If you are trying to accelerate, the car will fall flat on its face until the problem clears in about 10 seconds.
Please help, I am at a loss.