92 Buick Park Avenue transmission/towing

I tow a 2000# trailer and experience transmission troubles at times. I have a small external trans oil cooler mounted in front of the radiator and the tranny has about 75K so far. I'll be going down the highway at 60 mph and all of a sudden the car will start losing power/pulling ability when it downshifts out of overdrive. It doesn't help to manually downshift, and overdive is the only gear I can get it to continue pulling at 40-50mph. If I pull over and wait 5 minutes, it will work ok again until 20miles down the road. One time it started raining and the car got me all the way home (60 miles) without further problems.

Without a trailer it seems to work fine; although, on one extended trip I hit passing gear 200 miles into the trip and the car stuttered like it does when towing. I replaced the coil packs on my latest trip thinking it might not be a tranny problem; but, later, the problem presented itself again. It seems to help the situation by towing at night, which steers me back to a temperature problem with the tranny. The only thing I can think of trying now is to get a larger oil cooler. Prior to my previous road trip I changed the tranny's filter and fluid and didn't notice anything unusual in the pan or the old fluid's odor. Problem continues to exist, however. Any ideas? bump I've been towing with this vehicle for years and I believe it is rated for 2000#. I'm usually by myself when towing and have been 750 miles in each direction at least twice a year. The Buick is a 6 passenger vehicle which means it could conceivably have that much weight in the cockpit and trunk, which is different than tow weight. The brake system is capable at the GVW and probably a little beyond. I like towing with this vehicle because of the comfort level and stability at speeds. Plus, it's great to sleep in for a couple of hours when traveling at night. It has cargo coils and automatic levelers on the suspension and gets decent gas mileage.

My trailer is 8.5' wide and I carry an airplane (trike) which weighs 550# and sticks up over the car's roofline. This is a draggy scenario which is probably more of an issue than the weight. I'm guessing at the 2000# which may be less since the trailer has some aluminum in its construction, but the deck is 3/4Car Repair Talk's forum.