Is it the modem or router!

Before I schedule a service call (the modem is the cable companies) I thought I'd investigate the problem further! For almost four years I have not had any dropped connections with a Motorola SB5100 and a Linksys WRT54GS hard wired to the Desktop. Around midday yesterday I could not access the internet. I just attributed it to cable maintenance. However, at 4:00AM this morning it was down again. In both instances, the "PC Activity" light on the modem was lit, but wasn't blinking or only blinking intermittently. All of the other lights were on (except standby). I'm wondering that since the "Activity" light was on that the modem may not be the problem? Thank You!