This is my first automatic transmission, so I'm moving in unknown terain here.

I have a 700R4 transmission. The drivetrain is 100% stock.

If I launch the car from a standing start it flies through 1st, 2nd, 3rd. Needs about 7-7,5 seconds to reach 60 mph/100 kph which is close enough to factory statistics for a 20 year old car.

When driving 60 mph /100 kph and I give WOT the car shifts back and takes off as well. Shifts to 2nd, goes to 3rd. No problem there either.

The only problem occurs in one very specific situation.

When driving anywhere from 30 mph-50 mph /50 kph - 80 kph the car normally runs in 3rd gear. When applying throttle it shifts back to 2nd gear but does not accelerate.
Nor does RPM increase.

Can this have something to do with the tranny? Seems the only logical explanation, I just would not know where to start looking.

What irks me is that if the transmission slips RPM should increase at WOT.
It doesn't. Car just wheezes on at about 3000 rpm and hangs there for a few seconds. After this acceleration is back again.

I tested a few things while driving to get an idea before starting to take apart stuff.

After such an occurrence, when I accelerate beyond 50 mph/80 kph, let the speed drop back to 40 mph/65 kph and accelerating again the car picks up speed. Any lower and it doesn't. Starts wheezing again.

Acceleration at any starting speed below 30 mph works without problems.