Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I find that temperature related symptoms almost always ends up with an internal transmission problem. Pressure seals can leak when cold, but seal up when warmer. Or, worn clutch packs can slip cold but grab once the fluid temperature rises.
There's also the known issue with the trans fluid getting contaminated with coolant via the fluid cooler in the radiator. How did the fluid that came out look?