[justpassntime]

I was on my way home when I had this thought pop into my head, yes sometimes once in awhile I have an idea.

What about the catalytic converter? If it is coming apart or getting plugged it may do just what you are complaining about. It would run fine at idle or a little just above until enough pressure built up to cause this problem.

I had a couple of cars that did some strange things because of the catalytic converter.

One had totally broken loose inside and was sliding around. Ran great until the pieces slid back to block the exhaust from exiting. This one was hard to diagnose because it ran fine just sitting, but driving for a few miles or uphill would cause it. Once you stopped it would run fine again.

Another car just ran like crap most of the time until I pulled the catalytic converter off and basically dumped the dust and small pieces out of it.