My brother in laws 1973 450SL has devloped a unique problem. When the engine is started and put in drive or reverse, the engine rpms's suddenly drop way down but without the transmission really trying to move the car. Think of it like you left the parking brake on. It's just kind of bogging and bucking the engine without really moving.

Now if he puts it in park, and revs it slightly, the car will seem to want to creep forward! Not drastically, but if it had a manual tranny, I'd say someone was slipping the cluth heavily.

Any suggestions? His worst fear is the tranny or torque converter is gone. Another friend said check all vaccum lines, maybe somethings not engaging as it should.

Thanks for any help!