Hi!

I have 15" CRT monitor VISION DX-1564 (~1998), operating at 1024x768.
It supports refresh rate of up to 75Hz at 1024x768 and 60Hz at 1280x1024.

When you run a a monitor at the maximum supported scan rate for a given
resolution, you may find that it doesn't always run well, especially when
you get to the upper ends of the monitor's abilities. Sometimes that is just
the way it is--other times the monitor may have worn components that can't
handle what they once were designed for.

Have you checked to see what happens at 75Hz in lower resolutions?

What can be causing this problem?
The monitor is connect to S3 video card.

The video card shouldn't be a factor in this, so long as it basically works.
I have here a Trio64V2 with 2MB VRAM running a cheap-o Dell 17 inch monitor
at 1024x768/75Hz vertical. I didn't know that those early S3 video chips
were capable of that high a scan rate, but it will go to 85Hz if I select
that.

William