Transmission Shift Problem 1989 Auto

My son's car is a 1989 2 liter, auotmatic. head gasket was poor, we recently changed it and when the car was back on the road, it would not shift until it reached high RPM, (like 3500-4000) and then shifted hard. I checked the Chilton manual, it said the trottle valve cable needs to be rest after being detached (which it was for the head removal). I ran through the reset procedure, and transmission now shifts.

From 1 to 2 is normally smooth, and typically at the right RPM. Shift from 2-3 is slow, motor RPM increases about 500 (like a slipping tranny) but does not feel to slip, and shift takes 5 to 6 seconds. This condition is worse when applying more acelerator to the car. If you go slow, shift is normal through 1-2 and 2-3 . Takes just a second. as I would expected to.

When I down shift from 3 to 2 and then upshift, shift is smooth takes only a second, no RPM increase or feel of slippage.

No engine codes up. Tried resetting the ECM (Battery disconnect). Condition persists.

One thought is to change filter & oil, but trying to find a better reason for this problem, somehow linked it appears to how hard one applies the gas when accelerating.

Any thoughts at all are appreciated!