98 Pickup problem

Here's a problem I hope I can get some help with.
1998 Chevy K2500 350, 4L80-E, 4.10 gears. I have this issue when it's warmer outside, about 50? or warmer and/or when I've gotten the truck good
and warm, i.e. pulling my trailer, extended runs at 75+ mph. When I come to a stop, with the tranny still in gear, the revs will jump up and down 2 to 300 rpm's, causing the truck to lurch. If I put it in neutral, the surging stops. The hotter the day, or the truck, the longer the surging lasts and the higher the revs. I don't get a check engine light, don't see anything abnormal on the gauges, i.e. temperature isn't higher than normal. And very occasionally, I will get a hard 1-2 shift or 2-3 shift, like someone hits the underside with a sledge hammer. No tow-haul mode in this truck.
This is what I've done so far, changed fuel filter, plugs, wires, distributor cap, rotor, changed trans fluid. Cleaned the IAC valve, it's hole in the throttle body, and did an cleaned the carbon off the throttle body the best I could without removing it. Pushed together all the connectors I could see.
Dealer was of no help at all. They didn't know what to do without it tripping a code.
Any and all help in this matter would be great. Would really like to get this while it's still cold outside.
Thanks
Bald Guy
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