Direct from where Shaw brings its cables into your house. I bet the cables running from that box to your receiver are too old to handle the HD feed thats why it goes in and out.
I could be tottally off on this as its hard to tell without seeing the problem or troubleshooting, but that sounds like your best bet. What I would do is call Shaw tell them your problem and they will send someone out to look at it. I think Shaw gives free service calls so that will just give you an idea where to start.
If that fails go to Satellite...if you have an HD tv don't punish it by watching digital
