Simple solution may be to throttle the speed on your fax machine. You should be able to set the baud rate the 14,400 or below. You can also try turning off ECM if your fax machine allows it.
How is the fax connected? FXS port? In the past there was a bug with SCCP controlled FXS ports with a fax machine connected. If the above does not work, you could configure the FXS port to use H.323 instead of SCCP.
Hope this helps.
Brandon