Tell me if I'm understanding this correctly...after switching from direct connection from USB modem to connection through router, you havn't had any problems? If that is the case, I think that it is a problem with the communication via USB between the modem and your PC. USB is generaly not recomended as a Broadband communication tool. Some have even twisted the acronym to make it state "UnSuitable for Broadband." The actual issue could be caused by a whole spectrum of things. Driver issues could cause the system to become unstable in such a way that it causes its "vital organs" to shut down, thus causing no BSODs (provided you completed the process to enable BSODs, and are sure BSODs do not occur).
But to put it into a more extreme manner, it could be caused by anything as low level as a hardware issue. A failing southbridge, which I have witnessed before as more and more I/O components slowly failed, might cause the communication between the modem and PC to trigger a hardware system failure. That is far fetched, and would be a rare instance of a worst case senario.
I don't think a power failure by your PSU would cause that problem because it only seems to occur after using the modem's USB capability. In addition, USB devices generally don't pull much power from the port. It only draws around 4.75V - 5.25V at ~100-500 (I think) mA per device. That is more of a maximum really. Hence the reason why the USB devices usually use external power sources. As far as the supplying 5v rails going bad on your PSU, which would cause a USB power failure, would have caused so many more issues with other components that problems such as yours would occur much more often.
If I understood the posts correctly, which I may not have done due to coming into the post at a late time, I suggest either downloading new drivers for the modem, or switching to a diferent Broadband modem to see if the problem still exists.
I'm still considered a "noob" with computers, so you might want to wait and see if it gets certified from an admin. Even with that considered, I still like to share my extremely limited knowledge of computers.
