Car - 2001 Elantra, 5-speed manual, 63700 miles.

I bought this car with a P0422 (Main Catalytic converter below efficiency threshold). My state doesn't have emission laws. The upstream O2 sensor was fluctuating always, and the downstream frequently (about once 3 or 4 seconds) dipped below 100 mV. This seemed like the cat was genuinely screwed. I found the same car in the junkyard and replaced the cat, and the o2 sensors from the other car. The error code did not reappear.

This lasted for about a month. The downstream O2 sensor was also steady at 700 mV after warming up. Yesterday, MIL latched on again, and it was P0422 again. This time I made the following observations with the downstream O2 sensor -

1. During idle, it maintains around 700 mV and occasionally dips (about once in 10 secs).

2. If I VERY slowly increase throttle, it is happy and maintains 700 mV.

3. But if I push a little faster (practical for a real driving situation), it dips and recovers.

So I suspect that the problem is elsewhere, somewhere above the cat, in the fuel ratio. I don't know where to start, or if my suspicion is right.

There's always the question that the replaced cat was inherently bad, it coming from the junkyard. But that probability is less I guess, and given the fact that it was working well for a month. I drove a lot of city and highway during the month, so if it was bad in the first place, ECU would have caught it sooner.

Another observation that might help - sometimes (so random that I cannot qualify further) when I press the clutch (with 0 throttle), the engine rpm shoots up to 3000, lingers around there for a few seconds, and then comes down to normal idle.

Any ideas?

Thanks.