I have a 1995 GMC 1/2 ton 4x4 with the 4L60E auto. The tranny shifts well but sometimes it won't shift all the way into overdrive. It will shift into the half gear (thats what i've heard it called) between 3rd and 4th but no further. Then you can stop for a while, like an hour or so, and it shifts into overdrive properly. Also if you select 1st it will shift to 2nd on its own approximately at 10km/h and it doesn't hold the truck back in 1st if you let off the throttle. Like if you are going down a steep hill and want to use first gear to hold you back it just shifts into second. I have had the tranny flushed and that didn't help. There was no metal chunkies in the pan just the usual sludge type stuff and not even alot of that. There are no codes showing either.
Any suggestions?

 

I'll take a wild guess and say you may have a 1-2 shift solenoid problem. In the 4L60E transmission the 1-2 shift solenoid is on in 1st and 4th gear and off in 2nd and 3rd. It may be intermittent or sticking.