i'm not sure if this helps.
I've been plagued by a problem that the s3 driver do not support various ibm ramdacs. This is made known by the authors.
Chances are that you are facing a similar problem.
I've posted a kludge/hack which appears to enable me to start X windows
with the S3 driver on my diamond stealth 64 video card which uses a S3 968 video chip and a IBM 526 ramdac.
The kludge/hack/patch is on my web site at:
note that is is experimental, & i'm not warrantying it.
anyways, hope this helps,