My 04 Lexus 330 SUV also does this. In Kansas, never a problem, but on vacation to mtns, it was. Dealer has to “re-download” a computer program that tells the transmission to behave and it works fine on hilly roads after that. Lasts for 2-3 mos, then have to do it again. Dealer says car has “driving memory” that adapts to way car is driven (?)(!)
Make sure you are not either low on fluid, or overfilled.