Had a similar problem w/ 83 W126. It was brake related. I replaced pads, rotors AND BRAKE LINES. I heard or read that the problem can be related to any of those and all were changed. If your lines weren't changed, I'd think about changing them.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126