I've not seen anything like this before, or at least not where it's only happening with one specific site.
If you have your modem plugged into a power strip or surge protector try plugging it directly into an outlet to see if that makes any difference. And though the tech probably did so, please make sure that the coax connector at the back of the modem is finger tight. It doesn't need to be tighter than this, but it shouldn't be loose. In addition, please make sure that the power plug coming into the back of the modem is all the way in. Please be careful with this plug, as it's delicate, but just make sure it's not loose.
If the problem is still happening after trying and making sure of these things, please post a reply to that effect.