2006 M45 loses power & stalls, then restarts fine

[Captain Obvious]

Got a friend with a 2006 M45 that is giving her trouble. It usually drives fine, but sometimes it loses a ton of power on the freeway (slows down to 40mph, even at full throttle) or dies completely. She pulls over and restarts it, and then it drives fine again for awhile.

The dealer initially said it was bad catalytic converters and replaced them, but the stalling issues remain. The dealer has looked at the car a few more times and has no idea what the problem is. Now she's afraid to drive the car anymore (stalling on LA freeways is a treacherous thing), so it just sits in her garage. What a waste of a nice car....

Any ideas? MAF would be my first guess, but you'd think the dealer would have checked it. Thanks!

[built2makeit]

have they tried checking the battery? seems similar to a problem i had with mine

[ltrrtl]

I have an 09 and this happened to me at low speeds on the highway. The dealer re-booted two of the computers onboard and I have been good to go ever since. Dealer is in Pleasanton, CA.