For me, one of the few times it makes sense to send my cars to a pro and pay for one hour of their time, $120, is when I think it's going to cost me 2-3X as much time to figure it out.

Dare I say it...
send it to a pro and have him tell you what's wrong once and for all?

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio