Engine hesitates, idles erratically

I have a 1997 4 cylinder Camry with about 125k miles that's hesitating and stumbling on hard acceleration/deceleration. It makes me nervous because when it stumbles, there's a loud thunk that comes from the front end. Also, the engine idles erratically--it alternates between smooth idling and a rough clicking sound where the car shakes. I haven't noticed any decrease in fuel economy and the only code the OBDII gives is the P0420 for the cat, which I'm having changed next week.

I've recently changed the spark plugs and wires, and the air filter. The timing belt and both front driveaxles are also new. Aside from the bad cat, I have a few other known problems--a slow oil leak and a slow power steering leak, both of which I keep close tabs on.

Since my car hit 100k miles I've had to replace a lot of stuff, mostly normal wear and tear, but I've discovered that its hard to find a mechanic who knows what he's talking about unless you have an idea, too. That being said, any ideas of what might be causing this problem before I start hunting down mechanics?

Thanks in advance!