O. T computer problems

I wouldnt mind some insight from those with similar problems. It is my modem, works off and on. Yesterday, for instance I was offline most of the day because of it. But I have been having trouble with it for weeks now. It was free when I changed over to high speed but only warranteed for one year A.T. & T gave me numbers to their store . In going to their store I notice their modems for my system or the same replacement is $50. But on ebay I can get a new one for between $10 and $20 maybe on auction. There is one on a buy it now for $33 add the shipping takes that one to $40. There are some used ones there on bid for $.01 to $5 but I dont want a used one having enough problems with my used one. I just wonder if anyone else has had modem problems after a year and half use or longer. Is it common for them to fail. Does everyone turn their power off of the pc at night. I usually leave on the power, would that cause the modem to fail sooner. Just trying to see if Im doing something wrong I guess.

Steve
Martinsville, In
Heat zone 6
Sunset zone 35