You don't mention the monitor you are using, but it's most likely a LCD Flatpanel. Those have usually much stricter timing requirements on digital inputs then on the analog inputs. Many will only sync at exactly 60Hz.
You may want to look into the XFree86.0.log file to see if the monitor reported the correct parameters via EDID/DDC. Look especially for a section "Supported additional Video Mode:" which is normally the optimal mode to run the monitor.
The braindead Philips 170B (which I have) reports the capabilities of the analog input on it's DVI-D connector, so autoconfiguration is guaranteed to be always wrong.