First I am assumming you have completley checked for vaccum leaks, ie. checked intake manifold, pinched lines off etc. If your compression is O.K. then I would start by removeing the air cleaner and run it, check to see that the throttle plate is opening fully, and that it is centered in the body. Did your mechanic check fuel pressure? Also the coolant temp sensor could be faulty, or the wire to the sensor could be broken. I would also check ign. timming, although I don't beleave this can be adjusted? I could be wrong, If the ign. timming is O.K. Then I would go ahead and disconnect the exhaust forward of the cat. And run it to see if cat is plugging up, although you should be able to tell w/ vacum gauge... keep applying throttle and vaccum gauge will keep dropping without recovery until throttle is released if plugged. Check w/ cat connected then w/out.
Usually w/ plugged exhaust you hear a kind of low pitched sound from the exhaust, almost a grrrrr on accleration, like your pulling a lot of weight up a hill, and the engine is giving all it's got but going almost nowhere!
Although if it's been this way for several thousand miles you may not have notice a change.
Hey the parking brakes not on? Just kidding!