I don't think low compression is your problem. I have not heard of an M119 engine with low compression yet; and the head gaskets are generally not an issue with this engine unless there was a massive overheat.

Have you checked the caps and rotors for corrosion? I'd start there. Also, make sure you have the correct Bosch F8DC4 plugs (absolutely no substitutes!).

If you want to measure something, I would check the resistance of the plug boots to make sure they are all about 2 K Ohm +/- .2 K Ohm. Start from the wire on the distributor caps all the way to where the boot meets the tip of the spark plugs. It should be about 2 K Ohm.

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