Another question about cable modem problems?

I’ve checked out a recent thread on Fluther regarding this topic and discovered some helpful ideas and hints. However, since I’m not particularly computer/Internet savvy, my problem may be different altogether. In summary:

I’m suscribed to Comcast High-Speed Internet; the installation was performed 3 months ago; I provided my own NEW cable modem (a lower-shelf brand I’d never heard of); everything worked great for 3 months. All of a sudden, I began to experience connection issues which have persisted for about 5 days now. One minute the connection is slow and unresponsive, and the next it’s lightning-fast and perfect (i.e. It took 3 tries just to get to this “Ask a Question” page).

I happen to know that Comcast is not currently performing any maintenence on their network locally. My browser (Internet Explorer 8.0) settings are correct; I even reset them just in case. All of the diagnostic/repair tools I’ve tried seem to think they’ve corrected the problem, and I’ve even reset my modem a few times. I’m leaning toward the conclusion that my cable modem is faulty and only works intermittently. Any other possible causes?