Maybe your ignition coil is on it's way out, just a guess. In my experience, if that's the case, it will be real sluggish and then as the rpms increase (whether it's the increase in rpms that helps, or just that some time has passed) the car will pick up again, shortly thereafter it seemingly goes back to normal.

Fuel filter wouldn't be my first guess - as you pointed out, that would be weird as WOT it runs better. If it's been a long time/many miles since it was replaced, not a bad idea regardless since it's cheap and (I'm thinking this applies to your model) easy to replace.

If the ignition timing was too retarded, he'd likely experience the opposite - at relatively low rpms, the car would run better (have more torque) at partial throttle than at full, or close to it, throttle.

I don't have much experience dealing with O2 sensors, as far as diagnosing problems, but it would make sense that an issue there might appear to go away at WOT :shrug: