Widescreen resolution (1680x1050) doesn't work

I have a PC-Chips M810DLU mobo.
It comes with an integrated SIS 650/740 video chip (shared memory)

Recently I purchased a 22"W Monitor and wold like to set it to it's "native" res which is 1680 by 1050. Originally, that mode wasn't even available.
I have updated the driver and not I can select it, but the screen then goes blank for a few seconds (and then come back to the old res.)

1600x1200 works, so it shouldn't be a matter of enough memory. I have also checked the refresh rate and made sure it's at 60Hz.

Any ideas? Suggestions? The mobo does not have an AGP slot, so I cannot put in a different vid card.