If the cat has any air leak in it this will fool the engine into thinking its lean (your ecu says thi sis the problem) but in actual fact it is not lean (as the excess air in the exhaust is tricking the lambda) So your ecu will richn it up to compensate, but your engine will now run rich (and lumpy) I think the fault goes away at higher throttle loads as the pressure in the exhaust stops the ingress of contaminating air, and stops the fueling problem. All this is assuming the hole in the cat is the problem, BTW it takes a VERY small hole to cause this. The only other thing i can think of is that there maybe a broken wire to the ecu from the front lambda, and as also mentioned the air mass meter (but my money is on a leaking cat) let me know how you get on cuz