Desktops forum: Severe monitor flicker

I just got a desktop at a local used computer store with an AMD Sempro 1.2Ghz processor and 256 Mb RAM. The graphics card is a Silicon Integrated Systems Coporation
Sis_741_741GX_M741
Mirage graphic engine
software version 3.78
driver version 3.78
Video memory size 16 Mb
Video memory clock 133 Mhz

The monitor, which I've been using for years and has been working fine is flickering so badly, it's like looking at a strobe light. The only refresh rate option is 60 Hz. I tried lowering the resolution which made no difference. I updated the driver online. So I took it back and they said nothing's wrong with it. When I got it back, although the Monitor tab in the Settings shows only a refresh rate option of 60 Hz, the Adapter/List All Modes button showed that I could change to a myriad of resolution settings including refresh rates from 60 - 80 Hz. I changed it to 1024X768 at 70Hz. It looked decent after that.
I booted the machine up today, and it's back to where it was previously - with no other refresh rate options at any resolution to other than 60 Hz. I have no idea what's going on here. Any clues?