1. Have you tried a different ethernet cable?? That would be the first thing I would do.
2. Have you tried bypassing the router and going directly to the modem?
3. Have you tried plugging everything associated with this setup into:
a: A different powerstrip?
b: A different wall outlet?
4. Have you installed new network chipset drivers.
5. Have you restored your computer to a check-point before the problem started?