Modem keeps disappearing

Recently I have experiencing modem problems with my Dimension 2350.  It worked for over a year just fine and now every so often when I try to connect to my ISP it can't find the modem.  I get no dial sounds at all.  If I restore to a previous date when the modem worked, then I can connect.  Once I do that it works again for a while and then all of a sudden one day I boot up and it can't find the modem again.  When I try and trouble shoot the modem, it tells me it is working properly.  Has anybody seen this before