how can i find out if i have a problem

[14mearth]

my cable modem has been giving me trouble. It worked fine for almost a year but last month is has been running poorly ,and disconnects by itself, etc. Im want to buy a new one within the next few days ,but i fear it may not be the cable modem that is having problems. Is there a way to find out if it something wrong with my modem or it's somethign else that is causing the problem. I would hate to waste money on a new modem and cant get refunded

[degsy]

Test another PC on that connection.
Test another OS on that connection. (e.g. Knoppix Live)
Try another cable.
Contact the ISP and ask them to run tests on the line.