I think the problem is in the monitor EDID reported..
I think we should try to disable the EDID mode validation bit by bit..
You already have the horiz & vert freq entries for the monitor.
You can create a new section:
Option "ModeValidation" "NoEdidModes"
Option "UseEDIDFreqs" "FALSE"
Option "UseEDIDDpi" "FALSE"
You could try adding these above entries one at a time starting from the top one
The last king hit is this...
Failing this buy a PC monitor with DVI & VGA.
Option "UseEDID" "FALSE"