SLOW reverse engagement 89 560 SEL

[Kebowers]

A recently acquired low miles 89 560 SEL has a worrysome condition. Tranny works great in forward gears. However reverse is slow to engage and doesn't 'lock up' sometimes unless throttle is blipped. It then engages real hard, enough to spin the tires against the brakes! When it does engage, it holds full engine torque though.

Is something sticking and then unsticking?

Changing fluid and filter didn't help a bit. No gum or trash, only slight amount of black/grey sludge (1/4 teaspoon) and pan had never been off.

Help!