I have an 84 s10 blazer 2wd with a 3.4l swap. Just picked the truck up a few months ago, so I'm not sure what transmission is in it...I'm pretty sure it's a 4 speed w/ overdrive though. This happens about once or twice a day usually:

I'll be driving on the highway (usually happens around 60-65 mpg) and it will start shifting back in forth from (what I'm guessing is) 4th to overdrive like every second. Like it can't make it's mind up. To get rid of it I have to either let up on the gas or give it more gas....if you keep a constant pedal it will continue to do it forever.

Tranny fluid looks brand new, right on the level. What would this problem be, transmission or something else? It's getting really annoying, and is probably losing mpg over it. I've always owned manuals, so this is my first auto. Let me know how to get rid of this problem, thanks!

 

I had a problem like this...did you check the tranny fluid while the truck is warm and running? I was doing all sorts of things to figure it out. turns out it was a quart low on tranny fluid and had a pinhole leak in a trans cooler line that was the problem. If you dont check the fluid with it running, it will look like you have plenty in there. if you did check it with it running, then i'm out of ideas. GL!

 

Yeah, I double checked last night after a 30 min. drive. Let it idle, run through all the gears and checked it twice, dead on the full hot mark each time. So it's got to be something else unfortunetely. What could it be?

 

was it highway driving or city driving?

 

It's always during highway driving, about 60-65 mph when it happens. Getting worse and worse it seems like.