As an aside, the Physical Memory issue was a problem with windows 98 not with XP nor P4 itself. At least after the P4 4.03 update.

The specific problem, thereafter, with P4 -6 is a VM problem (Virtual Memory).

As a test, try setting your VM to manual (not Windows Managed) and raising it above 2GB or lowering it below. What is the outcome? By allowing Windows to manage it automatically, you can be inadvertently causing the issues. This would be especially so since you mention having a 750GB HD. Windows uses available free space on your HD to manage VM.

I always set my VM manually and never let Windows manage it.