Page file isn't going to cut. Istaria is too demanding on the hard drive. If you have multiple hard drives its best to configure the page file on the drive that Istaria isn't installed to.
How much memory (RAM) do you have? I'm pretty sure that 8GB figure is BS. Microsoft states that 2GB is the minimum for Windows 7 x64 and I know for a fact that it will run on 1GB albeit slowly. 4GB is the recommended amount.
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Most likely it is a RAM issue. A defective or partially defective module can produce symptoms like this.
I recommend that you test the memory with a memory diagnostic tool. (ie Memtest86+ or the built-in Windows 7 memory tester). Integrated graphics may reallocate system memory (unless disabled in the BIOS).
On Windows 7 to run the memory diagnostic tool, simply type "memory" in the Start Menu's search box. Click the entry "Windows Memory Diagnostic" and follow the instructions. The test will run after a reboot (before Windows starts).
This tool isn't very desciptive. Basically it will tell you if there is a problem or not. Memtest86+ is a much better tool.