"Priority" Troubles

I doubt this is in the right spot; if someone could move it to the appropriate sub-forum, I'd be very grateful.

So, ever since I got my Computer (last April), I've been having a problem, and the problem only happens in Guild Wars. Whenever I start up the game, I get 70+ fps (on all high settings), for about 5 minutes. After that, My computer sets the "priority" for the game to "Low:" http://i285.photobucket.com/albums/l...owPriority.png Now, I can manually set the priority back to normal, but it just goes back to low in about 3 minutes.
Some more info:
☺This happens on XP, as well as on Vista.
☺Also, I've never had this problem on any computer than this Acer Aspire.
I've done all of the diagnostic stuff with both Windows, and Guild Wars, but I still have no idea what is causing this.

Thanks in advance for anyone who can help.
It's probably something very simple, but I can't figure out what.

GW will automatically put itself into "low" priority if you minimize it. so, in the process of checking its priority, you yourself set it to low priority.

Any way to force it NOT to do so?

what kind of drop in FPS are you getting in game then? Also, what's your video card?

@ Moriz: Yes it does that, however the problem begins before I minimize/tab out of the game.

@Elder III: The Drop is from 70+ fps to about 10 fps, so it is quite a difference. :P
Also, I'm running a NVidia GeForce 9600m, along with an AMD Athlon 64 Dual-Core Processor, and 3GB of DDR2 RAM.

The problem probably has nothing to do with "priority". Most likely, the cpu or gpu is overheating. Make sure the cooling fan is running and blow the dust out of any and all vents and fans.

Now that you mention it, my fan has been acting strangely, as of late. Every so often, it stops, and will not turn back on unless I restart the computer.

EDIT: So, based on what Quaker said, I was able to find & fix the problem. it was the fan; there was a build up of dust in the fan (a situation I had completely overlooked). after removing the blockage, the fan now runs perfectly, and the CPU heat level is normal.
I'd like to thank everyone who posted here, and this thread can now be closed.

Thanks for posting the result - too often these threads just end and we never know if the problem was solved.