2011 MBP Randomly Slows To A Crawl, Forcing Reboot

I'm having issues with my 2011 MPB 8,1. My system seems to be randomly "freezing up" and slows down to a major crawl. The symptom can be described as like the system just slows down with commands taking 7-10 seconds to execute (Kind of diffult to describe sorry.). I'm forced to restart the system eventually (I've tried at one point closing as many running applications possible to see if maybe some sort of memory leak or high cpu was occuring, but everything in Activity Monitor was normal and that didn't solve the issue.)

Anyways i believe i've observed this happening during internet browsing, particually browsing flash sites (The issue has happened during usage of both Firefox as well as Google Chrome). This issue doesn't happen much (About 6 times total in 4 months time) however its happened tonight as well as a few days earlier so its beginning to occur more often. The past 2 instinces have produced similar logs in Console:

7/27/11 8:15:15 PM kernel WaitForStamp: Overflowed waiting for stamp 0x0 on Main ring: called from

7/27/11 8:15:15 PM kernel timestamp = 0x80000bb8

7/27/11 8:15:15 PM kernel **** Debug info for apparent hang in Main graphics engine ****

7/27/11 8:15:15 PM kernel ring head = 0xffc02c70, wrap count = 0x7fe

7/27/11 8:15:15 PM kernel ring tail = 0x00002c70 ring control = 0x00003001 enabled, auto report disabled, not waiting, semaphore not waiting, length = 0x004 4KB pages

7/27/11 8:15:15 PM kernel timestamps = 0x80000bb8

7/27/11 8:15:15 PM kernel Semaphore register values:

7/27/11 8:15:15 PM kernel VRSYNC: (0x12044) = 0x80000bb8

7/27/11 8:15:15 PM kernel BRSYNC: (0x22040) = 0x0

7/27/11 8:15:15 PM kernel RVSYNC: (0x 2040) = 0x0

7/27/11 8:15:15 PM kernel BVSYNC: (0x22044) = 0x0

7/27/11 8:15:15 PM kernel RBSYNC: (0x 2044) = 0x0

7/27/11 8:15:15 PM kernel VBSYNC: (0x12040) = 0x0

7/27/11 8:15:15 PM kernel kIPEHR: 0x78171003

7/27/11 8:15:15 PM kernel kINSTDONE: 0xfffffffe

7/27/11 8:15:15 PM kernel kINSTDONE_1: 0xffffffff

I have replaced the default RAM with two-4GB sticks maxing out the system at 8GB when i first bought my MBP back in March if that helps narrow things down. what I've done to try and solve the issue in the past is repairing disk/file permissions, verifying disk and ran both a standard and extended Apple Hardware Test, resetted the PRAM etc.

any help or insight into this issue is much appreciated.