I own a 92 z28 with 63000 miles on a 305 tpi. It has slight mods: hypertech chip, 160 thermostat, airfoil, k&N air filters and 4.10 gears and an auto trans with shift kit. I recently did a tune up on the car and the car runs really strong. My problem is that when the temperature outside is below 40 degrees when I first start me car the rpms bounce between a 100 and 500 rpm for about 5 minutes and then stops. when it's warmer than 40 degrees outside it dosen' t do this. Is this normal?

 

I took a look at the Hypertech calibration on my Tunercat software and they didn't mess with any of the MAT tables relating to fuel or timing (I bought one of their chips when I had a stock TPI system).

When was the last time you checked/replaced your MAT sensor? Maybe it's having trouble relaying the signal to the ECM at <40° temperatures. The TPI MAT location is a serious PITA to get at. But you may want to inspect it to make sure it's good.