You said that this started when it began raining. It's possible that you have cracked distributor cap. However, the symptom of upcoming rain and a cracked distributor cap usually is that the car will not start at all.

There are any number of things that could cause such a problem, but the rain you mentioned brought this to mind.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles