I'd say your starting on the right track by changing the fluid. That could make a big difference. Be sure they (you) change the torgue converter fluid too. I believe the B2 piston affects reverse first. Not sure, anyone? It's referred to as catastrophic transmission failure. That's enough to scare the **** out of ya.
sbr
'79 450SL
milanbraun/palomino
76,000mi.