Have you guys tried flushing the ATF and replacing it with new ATF? Do these cars have a transmission filter? If yes, try replacing that along with the ATF flush. Remember that the torque converter should have a drain plug for the ATF too, drain that as well (MB's have them). Valvebody cleaning might also work, but this is best done by a qualified gearbox mechanic. Take note of the ATF appearance when you drain it, and check the bottom of the used oil container for metal particles.

Worse case scenario, reverse brake band and clutches are already worn out, overhaul and replacement is needed.

My 2 cents. Hope this helps.