Something I just noticed above, you mentioned that you have to reboot the modem after bypassing the router to obtain an IP address. This is actually normal behavior therefore in itself doesn't indicate a problem. Have you experienced any issues while the router has been bypassed?
Yes. But let me clarify. When bypassing the router, I rebooted the modem and connected directly to the PC. Everything was fine for a bit, but then it lost connectivity. The PC would show the yellow ! near the connectivity icon indicating no connection. Disable/renabling the NIC would then result in it not getting an IP, eventually defaulting to a 169 IP.
Today I have noticed the Online light goes from solid to flashing, and later back to solid. This is new and was not occurring this weekend.
The Ethernet lights changing would suggest there some sort of problem going on with the local Ethernet segment, perhaps a modem/pc/cable issue. Can you try a different cable and see what happens? Perhaps it's time for a technician to come out and troubleshoot further.