seems odd, i have OD on the street but not on the highway? anyone ever seen this?

 

what do you mean you have OD on the street?

 

I actually had the same problem it would feel like it would lock up and hit od city driving but with any constant throttle input it would unlock and shift to third.
It was my tps I changed that and bam, trans works great!

 

Meaning anything under 50 mph i have overdrive, it shifts into 4th and then drops another few hundred rpms. it docent on the highway 55 and up pretty much. i don't have an external fps, mines built into the throttle body (drive by wire)

 

i just read this thread and no lie my 72 ls swap did a very similar thing last night, ive driven it around 500 miles around town backroads and such and it seems fine got on the highway for a little few mins and the converter locks up in 4th (drops a gear and rpms go up) the truck has a bench tune but still needs dialed in so, I hope is is something like the tps and not a trans problem or overheating problem, i just put all new lines and an auxiliary cooler.