2011 Chevy Malibu Trans(?) Issue

My 2011 Chevy Malibu (150k miles, LT) has a bizarre issue where if you’re driving in slow stop and go traffic it’ll lose power after sitting at a stop for a while, but it takes like 15-20 min of driving like that for it to happen. The car doesn’t stall, but if you hit the gas it barely revs up and won’t shift gears anymore. Putting it in reverse or neutral shifts really hard from drive.

If I’m in drive the car only revs up to maybe 1800 rpm from idling at about 800. If I put it in neutral or reverse the engine will rev like normal. I can get the car going maybe 15 mph, but the trans won’t shift up another gear so that’s about all I can get. The manual shifting mode also doesn’t work while it’s happening either.

It only happens in slow traffic, if driving normally it runs fine. Throws a generic transmission code (P0700 I think) when it does happen. If you turn the car off, wait 10-15 sec and turn it back on it’ll run normal again for a few min then happen again. Leave the car off for an hour+ and it’ll take 15-20 min of driving slow to happen again.

The dealer had the car for a week and couldn’t replicate the problem, so I drove it around their parking lot slow stopping a lot and got it to happen, took it right into the garage and they still don’t really know. Their best guess is something internal in the trans and want to tear it down, but dropping $4k+ on a fishing expedition is a little out of the question… Any ideas? I feel like it has to be something with a computer but I’m pretty lost, and seems like the dealer is too.