Monitor gets very blurry and distorted

Monitor gets very blurry and distorted, then
audibly "pops" off then back on (in the same way it does
when you change resolution settings) and when it "pops"
back on, it is similar to the effect you get when you do
the GAUSSIAN?????

Cannot pinpoint the reason. Doesn't seem to happen at any
certain time or during any certain conditions and there is
no documentation in the event viewer. Monitor is a Sony
Trinitron HMD-A400, about 2 years old. I am running Xp Pro
with NVIDIA GeForce4 Ti 4600. Unsure if it's a power,
computer, monitor, driver issue. I've updated all
drivers, reinstalled (clean install) operating system.
AGP aperture size is set at 128MB in BIOS. Resolution is
set at 1024 x 768, refresh rate is set at 60 hertz.