Nice car! I'd love to have someone's "throwaway" black/black SL600 anytime as a project...these M120's can be a beach, though the fact yours starts and runs is VERY good. Stay at it....

BTW, you may want to line up the SL 600 emblems on the decklid to spec...

Sounds like the 'biodegradeable' wiring harness might be suspect. Check there first (underhood areas, check for insulation cracking in joint areas, etc). Thats the best case scenario.

Though...I understand the M120 uses TWO ezl modules, TWO coils, one for each side. Check the one thats suspect, as well as the coil for that bank of cylinders, R&R and retest.

Best of luck and least of all, WELCOME to PP!