I have a 2001 Monte Carlo SS with 127,000 miles and the other morning my car started idling really rough. So I started to pull off and it was kind of "spurting" as I was getting to speed. I checked the codes and it brought up P0302, P0481 and P1404. After some research I ended up replacing the EGR valve. That got rid of the P1404 code. I know the P0481 is a cooling fan circuit, so that's not the problem. And the other code is a cylinder 2 misfire. Once I get up to speed it smoothes out but sitting still it runs real rough. Even when I put it in neutral when stopped its stops shaking. Does anyone have any idea what this could be?

 

I'm going to advise a tune up.
Plugs wires. You may also be experiencing a clogged/clogging catalytic converter. Could also be water in the fuel (condensation). In the gas.