i've got a '97 civic with about 156K miles - had it about three months now. It was running great for about almost two months, then it stalled on my dad once. after that, it was fine again. and then on a rainy day it just started shaking violently while in idle, and a couple of days after that, my CEL came on, and it stalled at a red light. since the light was on, someone lent me a reader, and it came up with the code PO420. Brought to my mechanic, who said everything looked fine and it ran fine. i didn't have trouble for a little while, and then the light came on again and it started stalling constantly, in idle. i'm usually at a red light, and when i press the gas pedal after the light changes the car jolts a little and then just stalls.
the cat is new, so should i just worry about the o2 sensors? could it possibly be anything else?

 

Start by replacing the plugs, wires, cap, and rotor. It sounds like the engine is misfiring, which is destroying the new cat.