1998 Trooper slips out of gear when going uphill

I have a 1998 Isuzu Trooper 3.5L with 109,000 miles on it. The automatic transmission was rebuilt in 2009 at 98000 miles. The transmission has always felt a little bit "chunky" going between gears - doesn't make weird noises but lurches a little bit. Is this normal? First slipping I noticed was going in reverse up a steepish hill. The truck went out of reverse and into neutral. Stayed in reverse if I held onto the shifter to keep it from going to neutral.

Next I was driving up a mountain and the transmission went from being in drive to being like it was neutral. Transmission light started flashing and had to turn around and go back home. Waited a few days to take it to the shop, during which time the transmission light didn't come on again. I took it to the shop today and they said they couldn't find any codes on the computer - that the computer only stores codes for so long. They did a service and checked the solenoids. They checked the engine mounts. Transmission fluid looked great and there was very little metal on the magnet. They say they have no idea why the transmission did what it did - that they'd have to have a code to pinpoint what the issue is. Does anyone out there have a better answer than that??

Thanks!