Hard upshifts ONLY when HOT??

My 1990 Buick Regal has had a problem for sometime now(glad I found this board). When the engine starts to get hot, and reach the near mid-way point of the temperature gauge, the transmission starts
upshifting hard, especially when going into 2nd gear.
I had the transmission serviced with new fluid and filter but that did nothing to help remedy the situation.
Could this be tranmission fluid viscosity issue? I figure the only difference between a cold and hot running transmission is that the fluid is "thinner" when hot, right? I've checked the fluid when hot and it still looks good ie. not brown or burnt.
Any recommendations?
THANX
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Maybe try playing with the fluid level? I know mine seems to work best at 3/4 between low and full. Even if its in the "good" area on the stick it will still shift hard (revs huge and SLAMS into 2nd)
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Try pouring some Lubeguard in to the tranny. It helped some people with shifts

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that is not really related to temperature, only appears to be. The problem is with the torque converter clutch failing to engage, and the hard upshifts are the transmission trying to compensate for "something slipping" with OBDII, you get a service engine light, I don't know what OBDI will do.
Try this: Drive around next time in "3" instead of "OD", which should keep the TCC from trying to engage. If the hard shift goes away, you found the problem.
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Isn't a 90 regal a 440t4?

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