'91 420SEL won't shift into reverse

[Morocco]

My '91 420SEL will only shift into reverse when I first start the vehicle, i.e. when the engine is cold. When it warms up, reverse does not engage and the shift lever just kind of slops through the reverse position without much resistance. Even when it will shift into reverse, I have to rev the engine to get it to "catch," and it slips a lot, but it's enuf to get me out of the driveway anyway. The fluid level is full.

Any ideas?? Thanks