hi, i have a 1994 k1500 w/t with an automatic with electronic overdrive. you can be going down the highway say 70mph and out of the blue it will shift down into 3rd sometimes shift back, hell others i smash the pedal take it to 80mph let off fuel and some time later it will shift back into 4th. or i can watch the tach and it's sitting at 2k moving down the highway then the rpm's will go up 2500,come back down. all most as if it were gonna shifht back down into 3rd. when i'm moving along at 45-55 haven't had the problem, just at 65 and above. some days not at all. i had the tranny checked out,and flushed since it was thier. they say nothing wrong with tranny everything is fine. the truck only has 80k mileage on it. i got it 6yrs ago with 42k miles on it. is their a vaccum line to tell tranny to shift down? i know the old ones had a cable connected with throttle that kicked it down, not this one. hey guys any help would be greatly appreciated considering this has me going insane and wanting to sell it from frustration thanks chad

 

I had one of my trucks do the same thing several snows ago. The trans in this truck is a 4l80e. Turned out to be one of the pressure bans, that controls the fluid rate.?. I think this what they told me at nthe time.
My truck progressively got worse over time then would not go into O/d at all.