Don't know if your model has the 'ice cube' relays or not, but the clicking and engine surge you describe sounds like the problem I've traced to one of these particular relays in my SL. I ran it down in the passenger footwell on the right side by feeling for it with my hand when the clicking occurred.

I ordered one and then pulled the cap off the one I removed. The right arm (contact) on the removed relay was offset and only making partial contact. I realigned the contact, slipped the cover back on it and stuck it back in the car. It has run fine for several months now, but only a couple days ago I noticed the clicking again. Could be the same relay or perhaps another. I'll use the same technique to isolate the problem again.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000