Sounds like the wireless. A "flapping" cable modem most likely would not be fixed by unplugging the wireless router although if timed right I suppose it might seem like it does. Either way a good cable provider can log into the headend and tell you whether cable modem has been flapping or losing signal. Usually you'll have a lot of problems with a cable modem that is flapping.
Are you in an area with a lot of other people using wireless? If you're using Windows XP, are you using Service Pack 2? The wireless support in SP 2 seems a lot more reliable to me. Also, if there's a lot of other people using wireless around you and you're on linksys router log into it and change the channel to 5.