Twice in the past few weeks I've had something random happen when slowing down to an almost complete stop. My rpm's will quickly drop down below zero (where the auto start/stop would be) then jump back up to a regular idle location. It happens very quickly. Probably within a second or less. It's happened while the start/stop has been turned on, and then another time with it turned off. Any ideas would could cause this to happen?

gmc 6.2 fwiw

I haven't had this happen on a vehicle with the auto-start/stop yet but... in the past, I've seen this behavior when the throttle body needed to be cleaned.

Usually on a vehicle with 70,000+ miles.
Driving for long distances in hot weather, everything is fine.
Then come to a complete stop - like at a red light in a small town - and the engine would die.
But, you could restart the engine and everything would run fine again.

I've had that situation happen on 3 different vehicles and the fix was to have the throttle body cleaned.
On our 1994 Grand Cherokee, it happened at almost exactly these mileages - 70k, 140k and 210k.

I guess with the auto start/stop, there's a possibility that the vehicle might just restart on its own.

I'd second what Hollywood said and check the throttle body. Gunk in there will certainly cause this problem.