Q: MacBook Pro 17" Late 2011 Crashing - Screen Glitch

MacBook Pro, 17" Late 2011

2.4 GHZ Intel Core i7

16gb OWC RAM, 1333 MHz, DDR3 (after market, went to 8G after purchase, and then to 16G a few months ago)

AMD Radeon HD 6770M 1024MB

OSX 10.9.4

I've been having 2 issues.

Occasionally the USB drops out - so my mouse (Logitech wireless USB) and Apple Extended Keyboard (wired) quit working.

This is usually solved by a reboot.

The more concerning is random crashes / freezes accompanied by beeping and screen glitches.

The laptop freezes, and often the screen glitches out like the attached. Squares, grids, etc.

Sometimes it just freezes on whatever I was doing.

I get 3 beeps, and 3 flashes of the power light. Over and over.

This has even happened several times overnight, when the machine was sleeping (once after a Time Machine backup).

I have run the Apple Hardware test (full test, took an hour or more) and there are no reported RAM issues.

I'm worried that this may be a GPU issue.

Any help appreciated. I have trolled through the AMD Radeon posts related to the iMac.

I do not have AppleCare, and we have a local Apple Retailer, but no store nearby.