No CELs?

How is it at WOT? If it runs fine w/ your foot to the floor I think(keeping in mind I'm not a mechanic) you can rule out fuel. An O2 sensor makes some sense, but I wouldn't expect it to have trouble just maintaining speed simply b/c it's running a bit lean or rich.

Is your alternator making any noise, sort of like a hair dryer?

If you know someone with a timing light, checking and adjusting the ignition timing isn't too hard. I'm necessarily saying it's off, but worth a check.
Apparently some people have just adjusted their distributor a cm or two, re-tightened the bolts, and taken a test drive to see if they noticed a difference... not sure how bad of an idea that might be, never done it myself.