rough fast idle at startup

I have a 87 surburban with new 383, new tbi 670 Holley,6000 miles,but 2 weeks ago, at start up it would not fast idle. the computer is tring to find it, but rpms fluctuate beween 1400 and 400 engine will die if foot not on the accelator while below the engine temp is below 135, then at 135, everything is back to normal(Runs great by the way). No codes. Mechanic check out O2, IAC. new fuel filter, HELP what to check next

did the mechanic check the throttle position sensor?

might have to chip the computer with the difference in the 383, several companies make the custom chips.