What would cause transmission to only slip when hott?

Usually this is due to a low fluid condition. Maybe 2 or 3+ quarts low. I've changed the tranny fluid on new acquisitions, including trannies over 20 years old that had never been serviced, and never had one "blow up" or fall apart because I changed the fluid. People make up ignorant stories that have no mechanical engineering prowess. As if leaving old fluid in anything is always better than changing it. IF the tranny is worn to the point where it isn't a fluid level issue, it likely needs to be rebuilt since fluid pressures will drop and there's too much metal residue in the inner workings like the valve body and pump and so on. 

ALL TRANSMISSION fluids have coefficients of FRICTION in them! Over time, they wear out just like engine oil does, and ALL OTHER fluids like STEERING and BRAKE and COOLANT. USE ONLY THE FATORY FLUID from the dealerships as nearly ALL oil change c enters us OFF BRAND FLUIDS to make more money from you! Same goes for FILTERS and other items as well There is NO SUCH THING as UNIVERSAL FLUIDS of any type! Even ENGINE OILS are focused to different types of engines ALWAYS USE THE SPEC NUMBERS. Like ROYAL PURPLE is a total rip-off. They have NO SPEC NUMBERS AT ALL< or didn't' heave them fr a while. The FACTORY FLUIDS are usually the BEST ONES. 

Change the fluid AND filter and hope for the best. When hot the fluid is thinner and the pressure drops. Could just be a dirty filter.

Change the fluid and the filter. That's almost certainly the cause.

First thing to try is the fluid.  If it's low, top it up to the right level, if it's dirty drain it and refill with fresh fluid to the correct level.  If that does the trick, there you are. Simple and cheap to do as a first step. 

If it doesn't do the trick then you do need to look further.

If you're lucky it's just a shift solenoid that needs changing, but that's just a wish and a guess. You need to pull the OBD2 codes. There may be a soft code telling what is acting up. Changing the fluid may not fix a thing.

find local shop at ATRA.COM for road test. don't replace fluid or parts till its been driven and you find out what the diagnosis is. since its a good bet that the transmission might need to be rebuilt

maybe you should get a mechanic to check it out

It's a pressure issue.

Buy two quarts of the correct trans fluid.

Drive it until it slips, that way you know the trans is warm.

Check the fluid per manufacturer instructions.

If it is low add ONE PINT at a time.

( I think it may take a full quart or more, but you can not over full it.)

Test drive.

If it shifts correctly you either have a leak or the fluid needs changed or both. If the fluid has more than 60,000 miles since last change, the transmission needs serviced sooner rather than later.

If it still slips. A fluid change MAY help, but start saving for a replacement or rebuild and keep an eye on the fluid level.

You're best bet is to go to the dealer and hope there is a factory TSB with a reflash for the PCM to take care of the problem.