General

Fps drops every few seconds

Well, hoped to avoid having to make a thread, but no such luck...

So, in short...Every few seconds my FPS will drop heavily, in effect it means my screen freezes for about half a second.
It also can coincide with me killing something, So every few times i kill a handful of mobs at once it will have a longer lasting lag spike alongside it.

Of note is the facts that my GPU usage drops whenever the FPS drops occur, where as my CPU use either stays normal or jumps (In the normal "Doing anything" lag spikes, and the combat lag spikes, respectively.)

My hardware is:
Motherboard: Gigabyte 78LMT-USB3 (Rev 4.1)
Graphics card: Nvidia GT 430. Drivers are up to date.
CPU: Amd phenom II x4 965 Black edition.
Four gigabytes of ram.
Im running windows vista home basic, SP 2, 32-bit.

If anyone could help with this issue, that would be much appreciated.