I have a bone stock 1990 LS 5 spd and recently it started doing something incredibly odd. Occassionally, I start my car and it will run normal for 3 seconds and then the RPMS take a nose dive. The car tries to stall. It will die if I dont press the gas pedal. It has only done it to me three or four times in the last three weeks. If I press the gas pedal in when it starts to bog, it will idle at about 1500 RPMS for 10 seconds then it is fine. After that it will idle and drive without any problems.
check the vacuum lines on the intake manifold, make sure none are pinched, cut, damaged or leaking, one certain vacuum line which can cause this problem is the vacuum line that goes to your map sensor, which is mounted on the firewall center slightly toward the passenger side, along with some vacuum purge solenoids.
you can physically check them, twist the vacuum hose, and pull them off to look at them all around.