Moden won't connect

On occasion, my modem will start to dial my ISP and then get "stuck" in a long screech and then I get a 678 error - the modem won't connect. Once this happens - I can't get it to work until I use System Restore and go back to a time when it was working. This computer is only 5 months old and it worked fine until about a month ago. My ISP says things are fine on their end, but it does happen occasionally and I can't understand why.

I've emailed support and they sent a list of things to do myself - I don't think it is a problem with the driver since it works most of the time, but occasionally it won't.

Any ideas?

TN