some engine problems can only be properly diagnose at the time the problem is occurring. If this is an intermittent problem, which it sounds as if it is, that is why they can't diagnose it. About the only thing you can do is continue to drive the car and when it does it again, try to get to your mechanic or the nearest one ASAP.
to bring your car to the same crappy mechanic before you get a new one?
It's the place with the best reputation in the area for service, and I've been very happy with other work they've done, so I wouldn't think they're just screwing me -- further, they're certainly not getting anything out of it, since the diagnostic checks are free.