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