If you have already addressed any problem related to weak power supplies, drivers or system pests, I would check to see if the modem has a feature to allow it to reset the PC from a remote location and disable that feature. If so, turn it of. It is also possible that the reset, if XP is your OS, has detected a fatal error. By design, it can reset your system rather than hang it. If this is the case, we may still be looking at a hardware issue. Good luck.
I am using a Creative Blaster 56K V.92 PCI "soft" Modem. It has been working fine but now shortly after it dials out, it will reboot the computer. Also, the modem isnt always showing up in the device manager.

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