If the plug boots are old, you might want to consider changing these out. The plug wires are different from American cars because they are solid core and seem to outlast the life of the car, so they don't have to be replaced on a regular basis. The boots do need replacing.

I know it's like shotgunning parts at the car, but you have to decide whether you want to let the shop take your money for the labor of troubleshooting (and maybe get it right), or replace parts yourself that will probably eventually need replacing anyway. It is a balancing act most of the time, but I lean in favor of taking an educated guess of what part is bad, usually with the help of the more experienced members on this forum (and I'm certainly not one of them).

Do you think it could be a coil? I haven't yet experienced a bad one on my 119's, but I do hear they are prone to fail. I'm not sure how the heat variable would affect the output of the coil though.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles