I just wanted to follow up on this thread with my resolution. Comcast was able to fix the problem. It appears that there was local interference/noise on Comcast's system from AT&T U-Verse. The line tech fixed it and now my ping-plot looks like the attached image. So if anyone sees a plot/symptoms that look like my original post and has ruled out their own equipment or the coax lines/connections coming into their house/apartment, then you should suspect signal interference.
Also, I want to say that the ping-plotter software is outstanding. It really helped me to diagnose the problem and convince Comcast that the issue was real. I have already started to recommend this software to friends and colleagues.
Thanks!