I've experienced this same problem and have had several clients experience it also.
After I upgraded to SR2 the problem went away (for me). I use a USR Sportster 33.6
d/f/v modem and had this problem with both my older 486 and pentium systems until
the OS upgrade.   Some clients still experience this lockup even w/ SR2 so I haven't
come to any conclusions as to the exact cause.  This was rather widely reported on
the Net a few years ago but I haven't seen much mention of it since SR2 came out.
You might try switching the physical serial ports (if the modem is in PnP mode, try
hard wiring it).  I've also thought about trying to change the FIFO buffer settings
and seeing if this had any positive effect; other than that you really need some
kind of monitoring utility that will record exactly what is happening at the time of
the lockup.  Sorry I can't be of more help.  Maybe someone else knows the solution.

--
Don Gray
NetCaliber Computer Services
(541)923-3255 [voice/fax]
http://www.netcaliber.com