I have a 1989 chevy c1500 350 tbi that when in park it idles up and down and no matter if its cold or hot it has no power untill the rpms pick up then takes off.
We have put a scanner on it and it showed all sensor readings to be normal.
There is a idle air control sensor that would need to be pulled out and cleaned along with the throttle body, and that port the IAC Valve goes into. It is best to take the throttle body off so you can clean under neath that port. May want to replace the base gasket also, for the TBI. It is located on the right back side of the TBI.
Good luck.... We replaced EVERYTHING over the summer... Still isn't working right!