Truck Has "no Power" - Only From A Dead Stop. Moves Away Sl

[dansierra]

I'm having a problem with a stock 1989 GMC Sierra 1500 - 305 engine. It has no power off the line even though I have the gas pedal right down. Sometimes it picks right up and goes from a stop. Other times it moves out slow with no power (not stumbling or missing) till it gets to 15-20mph then it's ok and responsive to the gas pedal. It goes through all 3 gears though even when it has the "no power" symptom. I don't have any problem once it is going. Kick down the pedal on the highway and away we go, no problem. This problem only happens from a dead stop, occasionally. I've had several things changed but no results yet. (map sensor, main distributor shaft, fuel pump strainer, fuel pump, cat converter, muffler, electronic control unit, sending unit). My mechanic gave up and thinks it might be a partially locked up torque converter. Trany shops don't think so though. Trany guys say this 400 hyrdramatic trany does not have electric torque control, it's mechanic/hydraulic. I checked the governor as suggested and it's ok. Problem all started when I had the trany fluid and filter changed (may be a co-incidence). The gm service center says my truck would not have a TV cable, it has vacuum control to the trany. Any ideas much appreciated.