Back to the clogged exhaust. I know the cat con is just a pass thru but if the guts of the muffler have colapsed/clogged you will get a similar symptom. Try pulling out the o2 sensor in front of the Cat Con and run it. It will throw a code but it will emliminate any development of backpressure. Had this happen on a 4.3L V6. Took 2 days to figure it out. When I removed the o2 sensor the exhaust was unplugged and the engine could breath. Worth a shot before you take it to a shop at $90/hour and up.