'Not the cat converter. It was brain lapse on my part. I'm my own worst enemy. When I did my tune up I took off the throttle body to clear the carbon and gum out of it and loosened the throttle position sensor to work the throttle plates. I didn't have to. I shouldn't have, but I did it anyway. To compound the problem I put it back together and forgot to tighted the screws so it seems the TPS gradually worked it's way loose. I didn't mark it's position before I fooled with it so I had to guess about where it was originally. Well, to make a long story short I turned it back to the position where the engine picked up a bit and wonder of wonders the surging and jake brake effect stopped. Of course now I'm thinking there is probably a way to set these things with a voltmeter instead of the ol' eyeball and ear technique. Any threads out there that might give me the method?