The monitor reporting 60hz when using a custom vertical total is intentional. The strobing persistence system is tied to the vertical total value, as refresh rates are (I assume by the VESA standard) established with certain VT values.
When a VT value is used that the monitor does not identify as belonging to a preset refresh rate, it reverts to the 60hz persistence values (0.167ms pulse widths per point of Strobe Duty; 0.167 x 100 = 16.7ms (the frame time for 60hz).
I "believe' this reporting behavior (in the OSD at least) was corrected in the XL2430T, but that's not the point (it still uses the 60hz values when at an unknown VT is active, like 1350, 1354, 1500, etc).
120hz missing from the EDID is an issue with Linux and some motherboard Bioses apparently. You were lucky that only 120hz vanished; some people have everything except 60hz vanish and then the monitor get reported as a "generic non PNP monitor." For some reason, some people have the EDID wind up completely corrupted and only get 60hz, but it hasn't been proven conclusively why. This is most likely an issue with Linux itself and how it talks to some motherboards' EFI/UEFI most likely. A 100% way to fix this issue is to use the mstar ISP flash device (VGA or DVI) or the RT809F flasher (VGA only), as that will flash the firmware with the hardware supplied.http://www.yoycart.com/Product/52781882 ... ment_tool/
or the RT809F flasher can flash the monitors also (I have them both, because reasons).