Looks like something between you and hop 3 is doing a pretty good job of creating an outage roughly once a minute. I'm not sure what hop 1 and hop 2 are (in a lot of cases, the cable modem is hop 1, but I don't often see hop 2 being blank as well).
Hop 3 is an AT&T IP Address (your cable service was probably originally AT&T before Comcast bought them, right?). So the problem is someplace between the computer that's collecting data (ie: running PingPlotter) and hop 3.
This could be caused by problems in the cable modem, problems in the AT&T network, or problems in your computer itself. If you have more than one computer connected to your network, it would probably be a good idea to try tracing from there and see if you get the same results. Also, make sure you don't have a service running on your computer that does something once a minute or so (like transfer a bunch of data). Some viruses / worms / etc might transfer data once a minute - in which case you might see symptoms kind of like what you're seeing.
If you have another internet connection available (like dialup, for example), try connecting to that and see if the same symptoms occur. If they do, then you know it's a problem with your computer. If not, then you can suspect that it's not.
The acuteness of your problem leads me to believe that the problem doesn't originate in your computer, but there's no way to know for sure without doing more investigation.
The next possible culprit would be some other piece of hardware in your network. If there is any router or hub you can eliminate between your computer and the cable modem, I would suggest trying this.
I suggest doing these things that you *can* affect because you really need to do what you can to eliminate the problem locally before affixing blame on Comcast. This is also the thing that you have the most control over. Comcast technical support can often help you with some of these same steps.
Next, you'll want to run PingPlotter for a day or so and make sure you're seeing the same symptoms during working hours. If you do, then this should be a lot easier to solve.
If you see these same symptoms all the time, call Comcast and ask them to send out a technician to look at the problem. Tell them you're getting periods of packet loss and really high latency once a minute or so, and you'd like them to send someone out to have a look at it. A technician should be able to help you isolate this problem - especially if it happens during working hours (and you can be there then), and you can show them the problem in PingPlotter when they arrive.
The problem, of course, will be that it won't be reproducible during a period of time when a technician can be there. Maybe it only happens at night, or maybe it happens during the day, but not the day the technician comes out. If this is the case, you'll need to show them some data (like the PingPlotter pictures you posted here), and ask them to help you troubleshoot. This can sometimes take a lot of work, and sometimes it's relatively easy.
One of the jobs of the support person is to understand if the problem is something in the wiring or something on your computer. This can sometimes be difficult, but being patient with the steps they suggest and being insistent that there is a problem is important. Be polite but firm - "I'm paying you for reliable service; I'm having problems with that service; I'm reasonable and would like to fix it; please help me fix it; I love your service when it's working, but I'm frustrated because it's not now;". With enough persistence, they will certainly help you get to the bottom of the problem.
Again, the problem is between your computer and the router at hop 3. PingPlotter shows this pretty clearly, but we can't see anything more detailed.
Just as an FYI, you can use the File -> Save Image... menu to save a .png file that can be posted to your website (or attached to your post here). png files are smaller and higher quality than JPGs for PingPlotter images, and PingPlotter will create them in one step - rather than having to go through a copy / paste operation like it looks like you did.
Please let us know if you find this advice helpful, and especially if you have success (or not) in getting the problem solved.
P.S. Feel free to take the text of this message, add a picture, and send it to Comcast. We have numerous success stories of helpful Comcast support techs helping solve problems very similar to yours.