Does the engine have any leaks or anything? Have you checked the relays (FPR, OVP)? I also wonder if perhaps it pertains to the coil pack. I have similar problems on my 87 300E. Just like you I have changed a lot of electrical components and still there is hesitation and power loss on occasion. If you do come up with the answer, be sure to let us all know as this has come up in some shape or form on the board before (with no real resolution that I have seen)

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87 300E
202k on the clock