thanks for your reply. The UseEdidFreqs was among the options I tested. When I did that, I found in the XFree86.0.log a message like "no valid EdidFreqs found" and the SoftEdids were used instead. It seems to me that X
simply failed to "extract" the frequencies from the panel.
I also tried to force 60 Hz generating a modeline with gtf but it didn't help.
If nothing helps I'll continue to use the 4191 driver which worked fine for me.