I can't figure out what's wrong, I was thinking either the ECM, prom, or loose wire. I'll be on the freeway and the light flickers a lot and then stays on. When it flickers the car stutters and when it stay on it acts like it isn't in Overdrive. If I smack the bottom of the pass dash the light goes off and the rpm goes back to where it should be. Although sometimes when I stop the car idles really low. I've cleaned and serviced the IAC tuned it up this last summer, replaced the fuel pump and relays and wire harness. And the computer puts up no codes. Do I just have to take apart the dash and start checking wire connections? It's an 86 Trans Am with TPI

 

You don't have to take apart the dash.

The ECM is located about where you're smacking the dash. Drop the panel under the dash (above the passenger feet), a couple of screws will allow the ECM to drop down. Check the connectors there, if any are loose or dirty, you found your problem.

 

Bing. Remeber to disconnect the battery first. Disconnect each ECM connector and inspect the contacts closely. Reconnect them, then remove and reconnect them repeatedly to help wipe the contacts clean. Make sure they're locked in place when you've finished. Remove the rectangular cover from the ECM and make sure the MemCal unit is solidly plugged into the motherboard.

You might want to leave the ECM exposed, reconnect the battery, then rstart the engine. Once the engine is warmed, you can "tap test" the ECM for loose internal connections by tapping on the component side of the ECM (the same side as the small MemCal access cover) with a couple of fingers while watching the MIL. If the MIL flickers or the engine stumbles, you may have an internal connection problem.