Wont run at slow speed. either full on or full off

Could possibly be the speed sensor malfunctioning. Have it checked or just run the idle screw up a couple turns for now to keep it running. Once it keeps running, turn the screw back out to a proper idle. And sometimes it's the AC speed up solenoid not working to raise the idle when AAC is on.

When you let up on the gas does it shift? If not it's the govener

I really don't think much is in your way. I believe the last time I did any of them on this vehicle, I had to disconnect the main connector/plug to get at the input speed sensor but other than that I'm pretty sure they are not real bad to get at. Below are a couple of pictures that may help. Also when unpluging the connectors, there is a little "rubber" seal that goes inside the connector to keep moisture and other things out. When unpluging the connectors be sure those seals don't fall out. Can't think of anything else. Pretty simple job. Hope this helps.....

Reset your ecm. Do this by turning your key to on position with a/c on " engine not running" then disconnect positive battery terminal wait 30 sec. and reconnect, Should clear out all codes from ecm. If this don't work may have to go put on the computer to diagnose problem. But i think this will help. Good luck and let me know how it go's.

I have a 1988 5.7 gmc that after start up idles fine runs strong under full throttle but at highway speed and under no load speed it sputters and at slow speed it is running rich i have done complete tune up rebuilt the tbi One concern i have is someone removed stock air cleaner system and the one they installed has no intake air sensor could this be the problem ?
IT CERTAINLY CAN BE THE PROBLEM. The air flow sensor if a VITAL part of your fuel delivery and emmision's system. Your mixture is controlled by a computer that uses the sensor data from that device. If you possibly can get the old one, put it back on. It is possible that I may not be totally right about this point but the operation of your engine is dependent on all the parts being there.
and also i have no codes and no ck engine light on.
There may not be code for a missing part. That system is not perfect.
I hope this helps,
Happy New Year,
Mark

Check the cable that goes from the throttle body to the small arm above the shift arm on the drivers side of the the transmission. Does the arm move as you press on the accelerator?

3 speed auto or 4 speed auto? Does the speedometer work?

slow speed is brown, fast speed is purple.
hope that helps.

Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

sounds right but check mass air flow unit and clean it out before buying anything