If you have an S4 and you saw the exhaust ports open while reving the engine from under the hood... tehre is a clog somewhere.

on the S4.. exhaust backpressure opens the ports.. if you have straight pipes instead of mufflers, then I would say that you cat is choked up. you should not get enough backpressure to open the ports on an s4 just by revving the engine in neutral. if they are opening, then there is a lot of backpressure in the system.

 

Fuel pump going bad? In-tank filter getting clogged? Check these things, my car lurched around then before I checked this stuff.