Inconsistent WOW fps from 75 to 3 and back

My fps can range from 50 to 75 then drop to 3 or 4. It will stay that way for awhile then back to normal. This can happen in the Barrens or in a heavy battle at Wintergrasp. I am not sure if it is Windows 7, the Nvidia 9100 graphics, or both.
Could the low dedicated video memory of 256MB be the problem?

Any suggestions would be appreciated.

Here are the pc's stats:

NVIDIA System Information report created on: 08/11/2010 20:29:27

[Display]
Processor: AMD Athlon(tm) II X4 620 Processor (2600 MHz)
Operating System: Windows 7 Home Premium, 64-bit
DirectX version: 11.0
GPU processor: GeForce 9100
Driver version: 258.96
CUDA Cores: 8
Core clock: 500 MHz
Shader clock: 1200 MHz
Memory clock: 15 MHz (30 MHz data rate)
Memory interface: 64-bit
Total available graphics memory: 2943 MB
Dedicated video memory: 256 MB DDR2
System video memory: 0 MB
Shared system memory: 2687 MB
Video BIOS version: 62.77.2F.00.06
IRQ: 22
Bus: FPCI

If you see something that is a potential problem, please let me know.
Thanks,
Q

yes GPU is = to a Geforce FX5500/6200/7400/8500/ 6-30FPS see if you can get a Geforce 9600GS/GT - GTS250 - Radeon 4650 - 5650 - or a cheap GT3XX geforce ranging from 256 - 1gb ram

My pick if you just playing wow is get a 5650 Radeon Or GT230/GT3XX or 9600GT Price range is in AU is $100-$200 if you play other games Go GTS250 (DX10) or 58xx for (DX11) but if your an nvidia fan and want DX11 g\wait for the GT4XX/GTX410 - 450

Thanks Apollo. I will look for those. Is the dedicated video memory of 256 MB too low for Wow play?

no its not if you disable Shadows to min and have textures low and Filtering around half
In some cases Shadows are a big hit on some nvidia GPUs like 8800GTS 320/64 - 9100-9400GS/GT