I also have this issue.
Macbook Pro Mid 2010 13"
2.4 GHz Intel Core 2 Duo
8 GB Crucial ram
Mac Os 10.8.2 (mountain lion)
Using Samsung 22" monitor with VGA to mini displayport as well
Usually it depends what I am doing but 30mins - 4 hrs I can expect my both of my screens to go grey (or sometimes flash green or purple) and totally unresponsive
Then I have to force shut down the laptop and boot up again. Rinse, repeat.
Tried the complete delete and reinstall of HD
SMC updates all done, but still happens
Anyone have any clues ?
You both may have the 'bad NVIDIA' GPU that shipped with some mid-2010 models. See http://support.apple.com/kb/TS4088?viewlocale=en_US&locale=en_US - you have three years from date of purchase to be eligible for a free logic board replacement, so get into your local Apple Store or an AASP as soon as you can.