Despite the fact that your only surfing is there a possibility that something is pushing your memory usage past 3.5GB?

I had a freeze problem where when memory usage past 3.5GB it would freeze. This generally would happen during heavy usage however in two occasions I had just started up and was surfing and had it freeze up with screen artifacts.

If your not already using maxmem to limit memory on boot you might give it a try. I would try maxmem=2048 (2GB) to be safe and if it fixes the problem nudge it up until you start having problems again. I have had pretty good luck using maxmem=3584 (3.5GB) with no crashes since.

If you find that fixed your problem you can add it to your com.apple.boot.plist in /Library/Preferences/SystemConfiguration/

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MOBO: Z77MX-QUO-AOS CPU: Core i7 3770K GPU: MSI N760 TF 2GD5/OC Case: Modded MacPro2,1
Memory: 32GB Corsair Vengeance (CMY32GX3M4A1600C9) Wifi: Airport Extreme bcm94321MCA BIOS: HermitCrab Labs H3A.816M
Monitor: AOC Q2963Pm 29" WFHD 2560x1080 21:9
Interests: KDE on Apple Darwin, Keeping it real with the command line, Helping those that help themselves