Check Engine light comes on, stays on for days, weeks...and then goes off! on 1998 Toyota Corolla

Catalytic converter replaced in Jan. 2011.
Both, all, two? O2 sensors replaced in Mar. & May of 2011.
Problem with check engine light coming on again in Sept. 2011. Replaced with new catalytic converter under warranty.
Fine for a few months, then check engine coming on again in Nov. of 2011.
Brand new catalytic converter put in again! Also, new O2 sensors.
Feb. of 2012...again!! check engine light. repair shop is out of business!! tried unsuccessfully to track down owner/mechanic.
Took my car to another shop. Mechanic says, you need a new catalytic converter!
aarrgghh...check engine light on again after being off now for about 3 months.

Asked by for the 1998 Toyota Corolla