have you tried disabling the built-in video controller in the
bios? Also, if you flicker is at the power-line frequency, you could
try changing the refresh rate to something faster or slower than the
power line frequency.

My computer came with on board video. I installed a 3D video card later. I was
able to change the settings in BIOS to not use the on board video and to recover
the RAM which was dedicated to on board video. Each BIOS is different so I
cannot tell you how to do it on yours. I just know that it can be done on mine
(HP).

Many thanks for your replies. I don't think i have disabled the
onboard video card but like you said i can't find in the bios where to
disable it. I've set up two video cards and disabled one of them.

Trouble is every motherboard is different. I'll have another look.

When at the boot up of the system it will say several things, press F10, Del Key
or maybe esc key to enter set up.

Once, years ago, when I was still working, we had a problem with an
HP/UX workstation that flickered badly.

Finally, we replaced the entire machine, and the flicker was still there.

So we replaced the monitor. The flicker was still there!

To make a long story short, it turned out that when the user had moved
the desk in her office, she had unknowingly put the old CRT monitor
(all we had back then) near a large AC power cable in the wall behind
the monitor, and the magnetic field from the cable was affecting the
monitor!

Unlikely to be your problem, even if you are still using an old CRT
monitor, but just a note to check for totally unexpected things...

Or put a fan next to your monitor too. Any thing that may cause a magnetic field
can do that. My four year old son can demonstrate that with a magnet and the
TV. A expensive demonstration of magnetic powers. As well of putting mass
amounts of icons on my desk top in no time. See daddy I playing a game. Yea I
see that and loads of the same icon all over the desktop.

Getting into the bios wasn't a problem but finding the bit about video
card is... FOR ME anyway! I can't find it on my other machine which is
a compaq.

Tinkering this afternoon the screen went some funny colours and i can
only assume that the onboard graphics is utterly shot! However, trials
this afternoon - i managed to find what i was looking for
1] onboard VGA 2] plugged VGA so 2 was selected and saved. Booted up
and it seems it started to load grub ... and didn't get much further.

Perhaps i've knackered that card. So i'm going to have to test it in
my other machine or get another card if it's damaged. Or ask someone i
know if he's had any more P3/P4s for chucking. That is one great
beauty of linux... one can offer a 'good home' for a slower ex-windoze
machine.

Having looked at pci cards - these days the only ones available are
pci-e. Would these work on a P3? I gather for linux i'm best off to go
for nVidia.

What BIOS is it? It is likely that somebody here has experience with it. It
usually says early in the boot process. Typical BIOS keys are the Esc, Delete
and the Function keys. They must be pressed shortly after the screen shows the
BIOS and RAM.