Sounds like you've got a mess to straighten out in the glow plug circuit. I think you need to examine it from head to toe and correct any and all 'fixes' by the PO AND correct the original malfunction. A wiring diagram will be priceless, but if you have a second car to look at you can probably sort it out.

As to the idle problem, you may need the upgraded gold rack damper bolt. A loping idle is usually the presenting symptom, but if you've also got motor mount issues and possibly injector issues, the loping could combine with roughness.

To get a smooth idle, you need new motor mounts (all three) and you need to have the injectors tested and balanced. Balanced means adjusted so that the opening pressures fall within a 5 bar range (3 bars to get really smooth).

__________________
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