This started several weeks ago and I (and a mechanic) haven't been able to clearly diagnose the problem.

I'm now thinking, as previous replies have suggested, maybe to take a look at the TPS. I'll have to search on how to diagnose it. Although it idles fine and revs fine without a load...hmmm <img src="/forums/images/graemlins/confused.gif" alt="" />

Your case may be the ignition coil. I've had coils act that way in damp weather.