is eather low on trans fluid,or the OD bands or clutches is going out.or it could be a valve body pump issue.
I was (and still am) having problems with shifting, tho probably not the same as you, trans would downshift and put my rpm's real low (like from 2k to 1250) and it would climb from there. when it got to about 1750 rpm I would get this rumble strip feeling like the trans was stuttering, so I took her and for some tests and the guy said it was ignition problems, that at a certain point for some reason the voltage was too low for the rpm's of the engine. Anyway so I've been trouble shooting got new spark plugs, cables and am messing with the ignition coils. so maybe check that out but definitly take her in for an electronic test (call around for free ones at trans shops)
I have come to the conclusion after much research that the temp solenoids within the transmission are worn out I will repair that in the spring for the time being the car is parked for the winter.
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