Tranny trouble any ideals

I have a 92 caprice wagon with a sick transmission... It starts by running fine well it does shift hard sometimes. After 20+ miles it starts to slip. If i continue to drive the rpms jump way up until it wont roll over 20mph (i havent tried to force it). If the car is parked it will fire and run fine for up to about 20 miles +/- then it starts over.
the 1st time - 5 min rest and ran ok
the following weekend drove it to a show about 60 miles 1 way no problems.
next week drove 20 miles slipppppp...no relief with sitting so tow it home.
drove it 3-4 miles today no trouble?

modulator? bands? any ideals
thanks in advance

the hoopty pic
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I would suspect the either a leak in the vacuum line to the modulator valve, or the valve itself. Change it first and see if problem is fixed. Cheaper than overhauling tranny. Also, I would not be driving the car too much until you solve this problem for if it is clutch plates that are beginning to wear...it will only get worse and you may end up with more trouble and cost.

Blade

another thought would be to try some Lucas Tranny oil treatment in as well...certainly won't hurt...

thought of the vac. but its all electric...even double checked in hayes manual. Nope its Parked til i find something out