M2 mysteriously loses power
 

My 2001 M2 ran perfectly until a couple of years ago. Now, after starting up and running well for about 10 minutes, the engine suddenly loses most of its power. The exhaust note drops down very low, but doesn't sound irregular. It runs like this, with barely enough power to keep moving on a level road, for about 30 seconds, and then just as mysteriously it jumps back to normal and from then on runs normally. Sometimes it takes much longer to recover, and occasionally I have to turn off the engine and restart it. All the computer code reads is "misfire." One mechanic said it was the oxygen sensor, and replaced it to the tune of $300, but that didn't solve the problem. Another said it's the catalytic converter, whose replacement is even more expensive. Has anyone had experience with this kind of problem? More important, has anyone had it--and solved it?!