If it gradually starts missing as it gets hotter, it could be a pre-ignition or detonation problem from carbon buildup on the pistons. I experienced this with a 119 engine in my 400E and it was completely solved by running two bottles of Chevron Techron in a full tank of gas. It is a cheap fix and you don't have much to lose by trying it.

Also, during periods of higher humidity, I have noticed in both my 400E and E420 that water does drip from the exhaust pipe. It may not be the coolant leaking into the combustion chamber, but the humidity of the air that the engine is intaking.

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