Before you do anything, find out what codes it's displaying at the DTC (diagnostic trouble code) socket in front of the battery (at the firewall). If you don't know about it, do a search on "code reader". I have the exact car and when it acts up, I can always read the codes to let me know what's wrong.

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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