Sounds like your car might be in Limp Mode. My car (03 WRX, close to stock) just lost the cat in the downpipe, which clogged the exhaust pretty good and it went into limp mode. It felt exactly how you're describing so, not saying you've got the same cat issue but that something, possibly that misfire is putting the car in limp mode.
Hopefully one of the Gurus on here can jump in and help you out more.