Tranny doing something weird in T/H

[Mrturner1]

I have a 99 Silverado and when in tow haul mode it takes off just fine, but then when I'm at speed and a light turns red in front of me I let off the gas and the rpm's drop to idle instead of decelerating with the truck, and then if the light turns green before I stop and I give it has it slips all the way up to whatever rpm it should've been at and then grabs and go's. This is a weird one, no slipping any other time, truck drives great otherwise and tows fin in D or 3 without T/H mode. Truck has 133,010 miles btw if that makes a difference and is 2wheel drive, 5.3