Tell me about it. My mid-2010 MBP has exactly this problem.
I went to Apple's Genius Bar (in North Cal) 3 times in the last couple of years for this same problem.
First time they asked me if I can reproduced the problem at the scene, and I failed. Since they said their diagnostic can't detect anything, they suggested me just wait for a software update. That was 10.6.8.
After couple of months, Lion was out, and an acknowledgement from Apple website acknowledged this problem and was said to extend the warranty to fix (the first of TS4088). So this time around I was more prepared, I knew I probably couldn't reproduce the crash at the scene (it was so random anyway, happened 1-2X a day during normal use), so I copy all the crash report, and also showed them the Apple's acknowledgement page regarding this problem and free repair. Also "luckily" I reproduced the problem on site this time! The guy seems confused and still asked me to deposit the computer for a couple days for some extended test. I was hopeful but after couple of days they called me and told me the computer passed their stupid test and they couldn't find any problem. Even though I can reproduced the problem and all those crash reports, they said it might because my computer is "old" and can't match the new Lion system well. Since it was pre-installed with snow-leopard, so it was out of warranty and no guaranteed it would match the new syste. I was of course totally ****** off but they refused to do any repair even though I showed them the piece of expensive crap was faulty.
After mount lion was out, it was still no fix. Now 10.9 is out, I tried and it crashed the same day I installed 10.9. So an year has passed I went to Genius Bar again. They basically behaved the same, refused to acknowledge the Apple's support page and wanted to do a test again. It passed the initial test and now they are doing the extensive test for a couple of days now. I have a feeling the result will be the same. Maybe I should send Tim Cook an email telling him how suck Apple service is and see if I got better luck this way? The "genius" lady basically told me she understands there is problem with my computer, she is not denying that, but if their test cannot reproduce the problem, they can't do anything about it.... =.= I guess that's their SOP, though it was totally against what their support website suggested.
http://support.apple.com/kb/TS4088?viewlocale=en_US
Very disappointed service.