Transmission slipping

99 Trooper, 100K miles, the transmission "slips" when starting off, ie, seems to go out of gear and rpm increases. By going to idle and starting off slowly several times, it gets underway. Otherwise, shifts okay. Does it when engine/tranny are hot from driving. Three episodes only as I have it parked! Dealer said it had slippage and speed sensor codes. I disconnected battery after one episode and when I reconnected, it drove off normally. Could I possibly have a TCU problem or is the tranny really shot like the dealer says? This GM tranny does have a history of computer problems, but I'm no expert and am looking for advice.

Update. Tranny is apart on bench. A flat retainer ring had failed and trashed most parts of the transmission. $3000 repair is better than the dealer's $4700 for factory rebuild, but after religiously changing the ATF, I was surprised that this GM tranny gave out due to metal fatigue.