HBONOW playback issues

Honestly, your best bet is to talk to the support folks at HBONOW.  There's not much we're going to be able to do from here.

Youtube has a great explanation of how video gets to you.  It's applicable to video delivery in general, and it's good to have a mental model of how this works when trying to figure out what's going wrong:

Google Video Quality Report

Here's what I can tell you:

You know that your computer is fast enough to play video, and you can see that in practice (although, if you're on WiFi, you should check to see if you see the same problem when plugged into ethernet -- you might just have a weird dead spot or some interference in the spot where you like to watch on your laptop).

You know that your connection to your Internet Service Provider is fast, and the connection from your ISP to *some* services on the Internet is typically fast enough to provide good quality video playback.

What you don't know and can't see directly, is whether the route that traffic takes from your computer to the particular server on HBONOW's content delivery network is fast enough to get you a video stream.  Content Delivery Networks are usually made of up thousands of geographically distributed servers, with the goal being to minimize the physical distance and number of network nodes between you and the video server.  By reducing the number of hops from you to the content server, the hope is that you get an optimal, low-congestion route for the traffic to take, but it's not guaranteed.

You could rule out bad/congested routing by trying out the service over a VPN connection.  If the connection over VPN is better than without, then the problem is the route traffic is taking from your ISP to the CDN node, and probably represents congestion on the ISP's side.  At that point it's time to report the issue to the ISP and hope they remedy it, or shop around for a different ISP.  Keep in mind that ISPs advertise the speed of the connection from you to them, but the true measure of a good ISP is how well traffic flows from them to other parts of the internet (i.e. how well they're peered).  I've personally had a 1Gbps fiber connection from an ISP that shall not be named that performed dismally in comparison to a 50Mbps cable connection when viewing youtube, because the ISP was peered poorly.

If the video feed is poor over both your ISP and VPN connections, then it's possible that the particular server on the CDN is problematic, which is where HBONOW would need to intervene.  They also should have good quality metrics on the feeds they're serving and awareness of problem spots in their delivery infrastructure.  They (or their CDN provider) should have the necessary visibility to troubleshoot any issues at the network level, although it's unlikely that a first-tier support rep is going to be able to do that.  At that point, it's really just something they'd need to get a network expert to look at more closely, so it's probably a matter of just reporting the issue and waiting for a resolution from their network operations staff...