I just fixed what sounds like the same problem, and I"m also in PA. Turned out that it was a faulty rad. cap, though it seemed fine to me. The first thing I'd do is get a new cap; they're only $4 and easy to change. Then, after it is warm, check to make sure there are no leaks anywhere. If that doesnt help, well, its only $4!
Oh, and if you need help on the way back, I'm in Pittsburgh, minutes from the trnpk.

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Paul
2004 E500 4matic; 72,000mi