My 86 pontiac fiero se 3.4 will run fine in drive or reverse but will miss until it cuts off in nuetral or park

Well considering that the Fiero never came with the 3.4 I would check the wiring harness. When they swapped motors the harness may have been modified and causing problems now. Need to know if it's missing from low fuel or bad spark. What 3.4L motor did they swap into the car?

As far as I know the motor came out of a camaro. All emmisions and wiring seem to be done correctly. It has the fiero upper intake and everything on it. its more of a lope then a miss. It was fluctuate from around 1100 rpm down to about 500 rpm it will do this several time a then cut off. But that's only in park or nuetral. The motor will idle at 1100- 900 in gear for as long as you want it tpo. I have checked for vacuum leaks as well and didn't find any

Ok. Has it been doing this since you bought it, or did it just recently start having problems. If it just started, the swap was done correctly, if it's been there since you bought it, then I have to assume they couldn't fix it and sold it.

I'm going to lean towards a fueling issue. Do you know if they are using the 3.4 injectors or the stock 2.8. And can you check to see if they "chiped" the ECM for the larger motor. If it's the stock PROM there should be a four letter code on the chip that matches a sticker on the ECM case. If they don't match it has an aftermarket chip or a custom burn.