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Last Ocotber, my 1987 240 wagon lost power on the highway and at the same time developed a rattle coming from underneath. My first fear was the transmission (220K miles), but my mechanic looked at it and thinks it's the catalytic converter. The problem is intermittent, loss of power, then okay for a while, then sluggish again. Rattle comes and goes. I finally brought it to a local muffler shop for replacement, but they checked it (back-pressure check and whatever else they do except cutting it open) and said it checked out fine. I have not heard the rattle since then (just a few days) and the car has power. My question is: Was this a pretty reliable diagnosis, and can I keep driving it until it gets so bad that it can be detected? Or will I get stuck somewhere with a clogged exhaust? What exactly do they do when they test it? Could it be that whatever was rattling got blown out? Any comments from someone who has gone through the bad cat process are very much appreciated.