I still have the shuddering, non-responsive problem. I started the car this morning, and it sounded fine at first, and so I took it for a drive. Then as I was stopped at a red light, the engine started misfiring and the RPM meter was jumping up and down as I sat waiting at the red light. Then the car stopped responding normally when I pressed on the accelerator. I could still get around, and the car didn't stall on me, but the engine was clearly not functioning normally.
I guess the next thing I'll check is the fuel pressure. I'm starting to think it could be a fuel pressure problem after all. The engine problems today had nothing to do with humidity or rain.
I'll go to the auto parts store on saturday and get a fuel pressure gauge. I'll post back with the results.