If it's not the rear mount, then another possibility is the accelerator linkage damper (little tiny shock absorber, should be mounted near the injection pump). When this damper goes bad, the movement of the engine and the throttle linkage introduces what you describe.

If that is not the cause, then I fear you need to have the clutch overhauled, be SURE to get OE parts from a dealer that are ordered using your VIN to get the correct disk.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC