Help - SH4 rebooting my system

My computer reboots when running SH4 then displays displayes the error "A hyper transport sync flood error occurred on last boot" during POST. I have the u-boat add-on. I've noticed it happens usually when I switch to periscope view or when there are heavy graphics like a ship on fire. I've searched other forums and tried some of what they suggested like replacing the power supply, increasing the NB voltage. I've also tried lowering the graphics in-game and tried a lower resolution. Also, this just started when I installed my new GeForce GT 520 video card. Before that I was using the onboard video which made the game too slow. I thought it might be overheating, so I also have been running with the chassis open.

This has me stumped and it's very annoying since I really like this game. Anyone have any ideas? Computer spces below. Thanks!

MSI mobo w/latest BIOS
AMD Athlon II x4 630, 2.8GHZ
New, 500W PSU
4GB RAM
Nvidia GeForce GT 520 w/ latest drivers
Windows 7, 64-bit

Edit: I'm also using directX 11