Q: Car lacks power

My car has been losing power. It started with just a stutter once in a while. It has turned into something much more serious. It has thrown the codes P0301, P0302, and P0420. I have replaced the spark plugs recently and the engine stopped acting up for about 50 miles. The ignition system was replaced 40,000 miles ago, the O2 sensor has been replaced as well. I thought it was the catalytic converter, but now I'm wondering if it's the fuel pump. Once the engine warms up to normal operating temperatures it runs smooth. I'm wondering if the fuel pump is causing the misfires which is causing the P0420 code to pop up. If it is the fuel pump why would it stop acting up once the engine warms up. It also tend to die whenever I have to stop.

My car has 164600 miles.
My car has a manual transmission.