Monitor HP 1502 and XFree

I'm keen to learn NetBSD, having been given a free, if severly under-powered i386 machine, but getting X to work is proving troublesome ...

The monitor manual says Vertical Sync at 30-61 kHz and Horizontal Sync at 56-76 Hz. My Video Card is a SiS 6326, with 8192 kb RAM (but 4096) for X' purposes).

I've set up XFConfig with these values, but the monitor doesn't like it: wavy, and oscillating background with a half-inch strip of blank space on the left hand side.

Reassuringly, the X log agrees ... "refresh out of range, hsync out of range, bad mode clock / interlace / doublescan".

Any clues for what I can do ?