Yukon goes limp to where you have to shift from 1st to 2nd and that's all you have

I have a 94 Yukon that I've switched the computers
and still no luck no one sells a chip stock chip for it
so when I when it's cold he'll run fine until you go
over 50 miles an hour then you'll lose overdrive
forth and third gear then you have to shift from 1st
to 2nd and that's all you got what could be my
problem