What could be wrong with my car?

I have a check engine light on, it keep saying it's the catalytic converter. We replaced that twice, it did say it was the 02 sensor replaced that twice. My car when it warms up or has been driven awhile feels like it is going to stall when stopped at a light. The rpm will also go from 1-3 and then below 1. We also replaced the brain, they thought it was that, there are no vacuum leaks had that checked out. I do not know what else to do.