System unbstable, help needed fx 5600

Hi,hope you can help and sorry for crass posting, but I think this is a
graphics or motherboard issue.

System spec is

400 W power supply
MSI KT4 Ultra Mobo
AMD Athlon 2500 XP Barton cpu
512 mb DDR 400 Crucial memory
MSI 128 mb FX 5600 Graphics
Maxtor 80GB 7200 rpm 8mb cache ATA 133 Hard Drive
Pioneer 105 DVD RW
Lite On 52x32x52 CD RW

system will rarely if ever reach the end of 3DMark 2001SE without crashing
out, if I up the multiplyer even by .5 it wont run anything that relies on
graphics.
I have the CPU and memory speed set to 166 (333) but funnily enough when I
run fuzzy logic 4 it shows 166 fsb in the window, but if I click on the fsb
button it shows 133 ?
As an aside, it also takes an eternity to switch off, the end program
dialogue box doesnt appear so I dont know whats causing it
Your help would be REALLY appreciated