What would cause a transmission to shift the way it's supposed to when it's cold and not shift out of first hear when it warms up? On a 1999 Chevy C2500 with 5.7 ltr

You could have trash in the filter.and it could be something as simple as changing the filter and draining the old fluid out . The best thing to do is find a transmission shop that dose free diagnostic testing and take it to them because they have all the computers and the proper equipment to test it for you. THAT way you won't be guessing. Ok hope this helps you .have a blessed week in Jesus name

Clutch grit in the shift solenoids, replace transmission filter and fresh fluid, retest !!

I have no clue