How did you set the timing? I had the same problem with my trans am. Because the computer adjusts the timing for you. But you have to have the t iming set perfect without the computer conected for it to idle right. Cause when you put it in drive and go the computer will compensate and adjust it.. then at idle itll put it back where you had it originally.
IF thats the problem just disconect the ECU from the distributor, set the timing.. let it idle a minute and reconect the ECU then it should be fine.