Intermittant Miss

My 93 Regal Gran Sport (3800, 62,000 miles) has an annoying probem. I hope someone can point me in the right direction. It runs flawlessly in city driving - smooth and powerful; but on the highway it has an intermittant miss I haven't been able to diagnose. :screwy: It's barely noticeable - worse when you accelerate a little to speed up or maintain speed up a hill.

I've run techron and seafoam in the fuel, replaced the plugs, plug wires, pcv valve, fuel filter, air filter and O2 sensor. Thinking it seemed like a minor vacuum leak, I tested and/or replaced all of the vacuum hoses. I've also replaced the injector o-rings, the o-ring at the vacuum manifold, the o-ring at the PCV cap, and replaced the transmission modulator. It idles at about 22-23 inches of vacuum which seems good, but the miss is still there. :banghead:

Any ideas?

Thanks very much.