Chevrolet Repair: Automatic transmission shifting, throttle position sensor, throttle linkage

Question
1994 Chevy S-10, 4.3V6, 4x2, 125000 miles.
Transmission serviced 120000 miles, pulled pan, was clean, no residue, fluid not burnt,changed filter and fluid.
Sometimes when starting off, level or hill no difference, engine races and transmission does not shift unless throttle is lifted.
May do this in subsequent gears until drive is reached and then works ok including downshift.  Transmission diagnostic says transmission ok,  Told may be throttle position sensor.  
Asked Chevrolet dealer about checking or replacement and he says is part of system computer and not servicable and that probably not problem. Says need to rebuild transmission for big bucks.  

Answer
Hello Don,
First...I am not a transmission expert...there should be one listed. But I will try to help if I can.
Many Chevy transmissions had what is called a throttle valve, which is connected by a cable, similar to an ols choke cable. It goes from the right front corner area of the transmission pan flange, and connects to the throttle linkage on the engine. it actually mechanically operates a valve in the tranny control valve, and keeps the tranny from shifting.
I don't know if they used them that late or not, but they did occasionally stick.

Another possibility is the seal on the filter pickup tube. It could be sucking air. If yours won't even shift into low till the throttle is lifted, that sounds more like it.

I assume you have checked the fluid level and condition since?

Van