First off, get rid of that POS if you can. I get angry just at the mention of SM56. A lot of times, the modem just has that motorola chip on it and the generic motorola drivers don't always work well-see if you can determine who actually made the modem and get their drivers.
It may be working differently under the two versions of windows for several reasons-the drivers are the first that come to mind. This is an HSP modem (offloading almost everything on the host processor), so another issue is basic systems resources and how the OS is utilizing them.
Do you ever experience disconnections while you are stressing your system? You may not realize how much bandwidth this thing is eating up-They usually won't even run successfully unless you have a P166!
Once again, get rid of it if you can-Even a new cheap HSP modem from ebay for less than ten bucks would be better than that thing!