JT Fax bus speed problems

[jep]

We just spent all weekend trying to get a Hayes JT Fax board
installed on a 386/25 machine.  We're running SCO 2.3.2 and
using the UNIFAX Fax drivers.  The software worked fine but
the FAX kept dropping digits when it tried to dial out.
After two days we finally tried slowing down the bus speed.
Now it works fine, but I want to run my machine as fast 
as possible.  Anybody got any ideas.