JR North wrote:

I ran the Windows Memory Diagnostic utility, both 512 sticks failed
Mats+. Ran the test with one stick installed-massive failure (over
6,000) on one and 45 on the other....

What brand chips are on the memory modules? Basically, anything with
heatsinks on it or a voltage specification higher than the normal 2.5V
for DDR (which is what I think the Asrock K8NF4G uses, not DDR2)
could be junk, even major brand modules. I recently got 2 bad Corsair
1GB DDR modules, and years ago I went through 8 bad Kingston 512GB
400MHz DDR modules, out of 12-13. Sometimes just changing the command
rate from 1T to 2T will miraculously eliminate all errors, but if your
BIOS doesn't allow setting this parameter manually, try an SPD editor,
but only on one module at a time (computer won't boot if the SPD data
is messed up in both modules). The best diagnostic is probably Gold
Memory 5.07, and when RealWorldTech.com evaluated several memory
tests, it beat everything but the expensive ones from Ultra X.
However version 6.xx has never found an error for me, and neither has
MemTest86+, unlike MemTest86 v. 3.xx.

If the mobo is more than about 18-24 months old, suspect any caps not
made by Nichicon, Chemicon, Panasonic, or Sanyo, such as OST, Ltec,
Teapo, or G-Luxon (they're back!), especially near any coils or the
north bridge chip. Also Chemicon KZG and KZJ series go bad, but
their KZE is OK.

If your monitor uses a CRT, the horizontal output transistor may have
fried. It's usually the transistor on the biggest heatsink. If it
doesn't have a damper diode inside, replace that diode as well. For
LCD monitors, a couple of experts have recommended resoldering all the
coil and transformer connections, in addition to replacing bad caps
and blown driver chips or transistors.