The reason I asked was because this sound like an engine problem. Yes, the catalist can be effected by this; however, remember this, that a catalist never dies...it is killed. It is done so by missfire that results from a bad firing from a plug, or wires, and in your case since your vehicle is rather old, I assuming high mileage, I would think that the rings are allowing by-passing of oil which is contributing to your problem. The best way to confirm this is to have a compression test done on the cylinders with a qualified Technician. If this turns out to be the case, there a couple of things that you can do. 1st have new rings put in, 2nd have a junk yard motor put in, low milage of course, or consider purchasing another vehicle. At this point use comsumer report to guide you...go at least 5 yrs old...buy from a middle age private owner instead of dealorship since most people go to dealors to get rid of their junk, and younger people don't get rid of vehicles unless they are having trouble with them.