I just went through this with my '03. Purge solenoid, fuel filter, PCV valve, none of that helped.
It was really stupid. Cracked, in two places, intake duct and a nasty air filter. When you gave it some throttle, it would literally partially collapse the intake duct until the cracks opened up. That's what I get for getting in the cab and pressing the pedal instead of pulling the plastic cover off the throttle body and rotating it there.
Pop the top off the air filter and leave it sitting up a bit and take it for a short drive. That will tell you if it's an intake restriction.