shadbush

when cold vert runs great. When warm however it sometimes stalls shifting car into reverse,also when stopping the rpm's drop to around 200 and the car almost stalls but than climbs again to 750. I think my omp is shot and have recently been premixing because I can't afford a new OMP. Lastly I failed my emission test . The garage installed a new O2 sensor bnut it actually failed worse. They said I needed a new cat. Would the cat have anything to do with these symptoms?. Any advise is greatly appreciated.