trany problem

I've read some postings about this subjet but I think I have a different problem. I replaced my transmission with a used one that cloncks in high gears when the weather is worm. after 30mn drive, the ride becomes rough and at around 50 mph it shifts twice. when I push on the accelerator i can't go over 70 mph.
when the weather is cold the transmission works great no clonk and the transmission oil doesn't smell. I went to change theoil and I have been told by 2 different mechanics that it was an interior problem even though it smells.
can somebody tells me what it is and what to do to fix it?

There are neumerous seals and gaskets inside that could be warping with heat, causing a leak, resulting in a pressure drop and funny shifting. If the leak is big enough, it will cause insuficient pressure to fully engage the clutch plates, and they will slip which causes a burning smell. If the transmission fluid smells like burnt rubber, you ARE stripping the clutch plates, and they will need to be replaced, as well as all seals/gaskets, and any other broken part. It might be easier to just get a new or rebuilt tranny, and trash the one you have. Until then, go easy on it. Don't *try* to go 70, ect. Never buy a used tranny that has a burnt smell to the fluid, or if the fluid isn't cherry red. Brown fluid = stripped clutch plates. After a while, you'll start losing gears.

Thanks! I gess I will go easy with this transmission until it dies. The transmission doesn't leak and the first 15 mn it shifts fine. the longuer the ride it become eavy like it wont shift and i have to release the accelerator like i drive it manualy and it shit. that happens in higher gear.