Experiencing nightly lag on desktop PC but not laptop (both hard-connected)

Is this a wired or wireless connection issue?
Wired.

ipconfig /all output:
Link.

Who is your ISP?
RoadRunner from Time Warner Cable.

What type of Broadband connection are you using?
DOCSIS 3.0 Cable with 30 Mb down and 5 Mb up.

What is the exact Make and Model of your Modem, Router or Modem/Router Combo?
Motorola SURFboard Gateway SBG6580 DOCSIS 3.0 Wireless Cable Modem (I disabled the IP Flood Detection option, as many people recommend).

What is the Name of the Anti-Virus, Security or Firewall Software installed from the problematic computer?
AVG 2013 (free edition) and Microsoft Security Essentials.

Issue I'm experiencing:
For well over a month now (I can't remember exactly when it started), I've been experiencing nightly lag spikes on a very predictable basis. This can best be explained graphically.

This graph shows my ping time to Google over the last 48 hours. There is a severe spike every night from around 7:30 PM to 11:00 PM. I usually ping around 30 ms, but during these spikes it goes as high as 160 and makes my connection unusable for anything more than web browsing. I have tried multiple modems/routers, called my ISP for support countless times, and even had a fresh cable line installed. Due to the spike occurring during prime time, I was fairly convinced that this issue was on my ISP's end and that I was at their mercy to get it resolved. Until tonight... when I randomly decided to run a ping test on my laptop (also directly connected to the same modem/router), and was amazed to see that it wasn't lagging (it's pinging a steady 30 ms while my desktop is currently spiking in the 150s). I'm almost certain that I tested for this in the past and saw that it was lagging; maybe I'm crazy, or maybe I did and it was an unrelated issue, but I suppose that's neither here nor there now...

So now I'm faced with the fact that this problem has got to be something on my end (a virus, faulty hardware or network card settings, background downloaders, etc.). I'm not tech-savvy enough to offer help on this kind of forum, but I am tech-savvy enough to be very careful with what I download, know how to check Windows services, run anti-virus, etc. I just recently installed AVG anti-virus tonight but came up with nothing from a full scan. I've closed all non-essential programs and run a netstat with nothing unusual appearing (while still lagging). This leads me to believe that it's either a well-hidden virus, something very weird with my network settings, or faulty hardware. Whatever it is, I'm still baffled by the nightly 3 hour occurrence of it, and I'm not really sure how to proceed from here.

Any help would be greatly, truly appreciated.