ok the first step compression test make sure you have good compression on all the cyl. if that is ok start the eng take an ice pick and a jumper wire with alligator clips clip one end to the ice pick and the other end to the eng. slide the ice pick down the plug wire boots and that will ground the spark do that one at a time and listen for rpm drop when you find one that does not drop you probably have a bad injector on that cyl. pull the injector out with the wire hooked to the injector put it in a clear bottle start the eng. for a minute and observe if is flowing fuel if all that checks out you probably have a plugged cat. if you shop around at muffler shops you can have an after market one installed for 80 to 100 bucks and you will need to replace the o2 sensor also if it has never been changed that maybe what caused this prob. the life span of an o2 sensor is 60 to 100k so its over due hope this helps let us know ....ted