Are the faxes good if send to another receiver? If the intel fax card receives from another sender, is it good?
Make sure you are sending at high resolution if the fax was created at high res. Once the fax has been sent, check the LogResolution value on the logentry (to insure that the fax actually went at the resolution that you defined, rather than a different resolution negotiated by the devices).
From the sender, try turning flow control on, by the manufacturer's suggested addition to the init string.