The first and easiest thing I would check would be the linkage adjustment to be sure its not holding the trans shaft from full rotation to park, causing the pawl to stay partially up causing a weak engagement, if that's ok, I would say your park pawl or the toothed drum it engages with are worn, slight rounding or tapering from wear will cause that to happen.

__________________
Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale