2000 Pontiac Montana 36K running hard & slipping - any ideas?

We've owned our 2000 Pontiac Montana van for 1 1/2 yrs, purchased at 21K. It now has 36K miles, and about 4000 miles ago the transmission
seemed to start slipping lightly, seemingly when the gas guage was low. The fluids were changed and nothing could be found. It now seems to occur when the engine gets hot after a bit of local driving or on a hot day. We'll hear a light high pitch whirring sound, and then when the gears shift its a rough jerking. I've seen one post about this from a couple years ago, and am wondering if this is a problem with the Montana in general and what I can do about it. It is now time for another oil change so what should I have the mechanic look into? Thanks
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I had a similar problem with my 2002 Montana. Started noticing a very slight slipping. Took it to the dealer and they ended up replacing some sort of relay in the tranny. They did this under warranty. They had to drop the tranny to get at this relay. Sorry I don't recall the exact name.
Marbo

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name.
I don't drive an automatic, and never will as long as I have a left foot to work the clutch, but would this be the lockup part of the tranny? The part that changes it from a fluid coupling to a fixed coupling?
I seem to remember something along this line on Cartalk on NPR a couple of weeks ago.
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