Even a bad cat wouldn't cause running problems unless it was clogged up, you would be getting a lean o2 code from that at least. My bet is you have the opposite. And an empty cat would show SES but it won't effect performance.
Go to walmart and buy a $15 OBD-II scanner and report back with codes.
My bet is fuel regulator and/or MAF.
Fuel reg is hard to test unless you have an O2 code. MAF is easy to test just disconnect and if the problem goes away or gets much better buy a new one.