89 Cressida trans drops out of high gear - likely cause?

Have an 89 Toyota Cressida. About a week ago I started experiencing two symptoms pretty much simultaneously. When cold it was reluctant to upshift. When hot after about 20 mins it began dropping out of 3rd/Overdrive - i.e. engine would race, clearly the trans wasn't in gear. I could manually drop it into second but wouldn't upshift back into 3rd/OD. Pull over, turn the car off, turn it back on ran okay for a while, would dump out of gear again.

I got it home, pulled the pan on the trans, replaced the filter. Flushed the trans by disconnecting the outlet line to the radiator and letting it flow into a bucket while pouring fresh in. Probably ran about 16 gals through the trans, with the goal of purging the trans and torque converter to get all the old fluid out. Admittedly it was overdue - there was some debris in the filter, some gray sludge in the pan, obvious buildup of particles clinging to the magnets in the pan. Purged it again the next day with probably 12 gals.

Both originally mentioned symptoms disappeared for a couple of days - shifted crisply even cold, no dumping out of high gear. Then tonight on the highway it dropped out of high gear again. Pulled over, put it in park, then back into Drive and it was okay again. I'm guessing it needs another rebuild, was done once about 8 - 10 years ago, got run dry once due to my being neglectful about a not tight enough pan.

I'm still curious, what particular component(s) in the trans do you think is causing this drop out of high gear symptom? Like I say, it stopped doing for a couple of days ago after a fluid transfusion but whatever caused it is still going on even if less prominently.

Thanks.

replying to brassplyer, miguel wrote:
Doesnt the cressida have the same engine and transmission as the supra?

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