Formula Z

Swapped a 0411 pcm into my 98 ls1 firebird using the 98 harness. Also swapped the 60e for an 80e. Car runs and drives when in any forward gear or reverse. Put the car in park or neutral and it stalls, but if I keep my foot on the throttle I can keep it running. I've checked and double checked my pins and can't come up with anything. Also my fuel gauge and speedo don't work but that's another thread for later unless you guys think it plays a part here. Thanks for any input