I agree with having the injectors checked / serviced. Other possible culprits: worn / dead engine mounts (three: two up front on the sides of the engine, third is at the back of the transmission right before the drive shaft.

Last place: rebuild the delivery valves on the IP, when the copper crush washer inside gets worn, they can cause a rough idle.

__________________
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