Automatic transmission gurus?

[WrenchMonkey]

The 4L85E trans in my 02 Burb has started to run hot, when towing the same 6000# trailer I've pulled for years without issue.

The weird thing is, it seems to be strictly speed-based. Up to 60 mph, I'm fine. Up and down hills, stop and go traffic, no issues at all. Stays about 200-210.

Above 60, even on flat ground (~2200 rpms I think) the gauge will slowly creep up. 220, 230, 250. Once it's hot, I have to get back below 50 for it to cool. And it does, cools right off. Between 50 and 60, it'll maintain temp, neither heat nor cool.

It does have a tranny cooler, and I could just swap that for a bigger one. But it seems like there's an underlying cause. One, I never had this problem before. And two, it's not just the transmission working hard. Because surely it works harder (and should generate more heat) pulling a mile-long hill at 50, than flat or even downhill at 70.

I know eventually the torque convertor should "lock in," but I don't know exactly how that works or if that could be my issue...

Any advice?