rough running C280 Fixed!
Thanks to all of you who tried to help me diagnose my poor, sick C280. After replacing plugs, wires, end caps and OVP relay, it turns out that Arthur Dalton was right in his initial diagnosis.
It was - in fact - the dreaded wiring harness. And, of course, my friendly 280 was WAY out of any warranty. However, found a great tech in Oklahoma City who treated me fairly and did the job in less than 3 hours. Will highly recommend them to anyone in the OKC area who is out of warranty and needs a reasonbly priced, master MB tech (17 years experience).
Thanks again to everyone.