Frames Per Second problem

Hey guys.
There is something that is really confusing me about world of warcraft and its graphics.
Sometimes my frames per second is really good, like 170 on lowest graphics, then I can turn my wow onto full settings and average 80-120 frames per second.
However on other days my frames per second on minimum settings cap out at 60 and wont budge any higher. Also my wow jerks a bit (so its not running at 120 frames and only showing 60).
Today my frames per second is capping out at 30 (minimum graphics and jerking slightly) and then when i put my graphics to maximum I only loose about 10 frames.

I do not multibox anymore (i used to dual-box a frost mage and elemental shaman) but right now I find it more productive to do two separate things at once ( Raid kara on shaman, farm primals on mage, etc)
However multiboxing will always be in my heart and I always try to help out the multiboxers on my server.

Anywas, I was wondering if any of you maybe had an answer as to why this might be happening.
*It is not due to background programs or processes.
*I do not have any viruses or spyware, or addware, i update regularly and scan every night.
*I have not changed any computer graphics settings or changed any hardware.
*I havent installed any software that might be affecting it.
*Also I do have 3 gigs of DDR2 pci-express (Running windows vista 34 bit)
*I mainly noticed this jump after i upgrades from 1 gig to 3 gigs of ram...not quite sure why. Is it possible that vista recognised that and is trying to use up more for more processes? I have all windows vista's widgets and what not disabled, im running on the windows classic theme etc.

Thanks in advance if you are able to help me :)

if it is stuck at 60... check to see if you have it set to hook to vertical sync (don't have exact wording with me). it's under the video tab in wow

Thanks for the reply. This is another interesting thing. I often have to alternate between having Vertical Syncing ticked, Vertical Syncing and Triple Buffing ticked, and having them both unticked due to screen tearing (happens like once a day) - as soon as I change one of those settings the tearing goes away again.
Do you suggest that I have both Vertical Syncing checks and Triple Buffing ticked when ever possible?