One of the things you've mentioned that rather stands out is your mpg.
I've always been pleased with my mpg (around 23-25) but 28? I believe you, because I once hit 28+ on a road trip that was almost all downhill, with a tailwind.

So anywho, I have a thought.
Have you pulled & checked your new plugs? Look to see if there are any signs of higher then normal temps.

You've mentioned your theory, but I'm curious if whether or not, your ECM/fuel injectors are injecting the proper amount of fuel. ie; maybe it's running too lean.

It's your high mpg, hesitation, and o2 light flashing that made me think of this.

luck

Travis