Full Size LeSabre Estate Wagon Downshifting Wrong

[tfvesquire]

My dad's 1986 (yes, '86!) Estate Wagon (5.0 ltr V8) has a problem when downshifting. Performed a full tune up (air & fuel filter, oil change, plugs, cap, wires, rotor, PCV, etc.) It runs great, but after you are driving it about 45 mins, it starts to shift weird.

Basically, whether you are running it on the highway in OD or around town in D3, when you go to slow down for a stop, the car RPMs almost stall out as it downshifts each gear until you stop completely. Then, when you go to accelerate slowly, the trans will run through gears 1-3 too quick for the engine to get the car up to speed (sort of like if you had a manual trans and shifted 1-3 without letting the RPMs of the engine get the car going, Then, it basically lopes in 3rd gear until the car gets moving to 35-50 mph. Runs fine and smooth otherwise.

I don't know if this trans (factory 700 R4 I believe) has an internal or external shift module and/or whether it is vacuum controlled. My guess is that the problem is caused by the valve body or some electrical module inside the trans heating up and then it starts to shift like crap.

Any idea of what to check before sending my dad to AMMCO Trans? Could it be a simple clogged trans filter and need basic service? I don't think the trans is ready to die because his other estate wagon did the same thing from time to time and had about 355K in the clock. This one only has 110K.

Any info would be great. Thanks everyone and keep those threads going for the DIY people!!

Ted, Chicago