The plats are a no-no. Use only what the owner's manual suggests.

Your fouling problem will eventually return. The rear most plugs are fouling first, no?

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E