Thought I might provide an update:

I contacted my ISP, Mediacom, after monitoring my connection for awhile longer and gathering more data. They said that they saw some problems with both the downstream and upstream, so they scheduled an appointment with a technician.

Sadly, the tech that came out seemed incredibly incompetent. I'm not even sure if he used his cable signal monitoring tool or if he just looked at some stuff on his phone (while connected to my wifi). He complained about his job half the time and didn't care one iota about the data I had captured with PingPlotter. I think because he didn't understand its relevance. He himself said that he lives in the sticks and I quote "doesn't have internet". Great.

I posted on a local facebook computer hardware buy/sell/trade page about my issues and someone responded. Turned out that he works for Mediacom and is apparently a bit higher up than the techs that normally make visits to customer's homes (he works on the equipment outside). Much to my joy he offered to help troubleshoot my issue and today it seems that he may have fixed it. Here is a snapshot of PingPlotter of today from when he started working on it up until now:

http://i.imgur.com/sMZKlWu.png?1

As you can see, the consistent ping spikes have stopped and my ping is now much more reasonable.

The moral of the story is this: Try to get in touch with someone who works for your ISP. This guy was even on bereavement leave when he contacted me, I couldn't believe it. Suffice to say that I'm extremely thankful for his help.

If it's of any use, he said that the MER was dropping to 28-30. He sent me a picture of his tool before he started working on it: http://i.imgur.com/y8MU3pM.jpg. And here's a picture of his tool after he had replaced a couple mods: http://i.imgur.com/c3EHrXm.jpg. There is still a small drop occurring but the connection is already much better.

He also said that the MER was till dropping after he replaced some equipment (a mod, as he called it), but now it's dropping to 35-40. Google says that a higher MER is better, though the tech said it shouldn't be dropping at all. Anyways, there you go, problem solved. This is what PingPlotter can look like when the MER is dropping, and it manifests as shown in the original post.