I had the same problems last week with my 3.4 DOHC Fiero. Major misfire, but only under load. Turned out that the #2 spark plug wire was corroded from rain water dripping off the back window and getting into the plug wells. I replaced the wire and the car is now running better than it has all summer.
I also made a quick, temporary cover to prevent water from getting back in there. Not pretty, but it'll keep water out till I come up with a more permanent solution.