If resetting your modem and not the Mac fixes it, I'd blame your modem as the most likely culprit.
The next time it goes out, just restart the Mac and see if that fixes it, if it doesn't, again I'd blame the modem.
In that case, the first step I'd try is leaving it unplugged overnight to see if it resets it. Also check to see if it has a reset button instead of just unplugging it.