Constantly losing connection, huge bandwidth spikes

Hi all,

I've been having connection issues the past few days. I do have an internet connection, but it drops out every 2-3 minutes for a minute or so at a time. Comcast was out here today and replaced our line, the tech said the signal was poor beforehand. Afterwards there was a noticable improvement in speed (actually getting the full 100 mbps now!) but the issue is still happening. I think I've narrowed it down to the modem, or an external issue like a DDoS attack. I have no idea why anyone would want to DoS me, since I don't do anything. But nonetheless, it's a massive problem for myself and my roommates.

Our network setup is Wall > Modem (NETGEAR CMD31T) > first router downstairs (NETGEAR R6250, running the stock firmware) > second router upstairs (NETGEAR WNR2000 v2, running DD-WRT v24-sp2 from 12/2010). I can ping/get into both routers and the other machines on our network without issue, so I really don't think it has to do with the routers. The first router is getting a valid IP from the modem.

Here's a log from Net Uptime Monitor, I've been running them pretty regularly and they're all about the same:

Net Uptime Monitor Failure Log (NetUptimeMonitor.com)
UNLICENSED TRIAL VERSION
 
=======================================
4/4/2016 22:56:41 PM	Log Start
---------------------------------------
Failure Start      	Length
4/4/2016 22:56:44 PM	0:00:20
4/4/2016 22:58:45 PM	0:00:36
4/4/2016 23:01:44 PM	0:00:36
4/4/2016 23:02:27 PM	0:00:33
4/4/2016 23:04:39 PM	0:00:27
4/4/2016 23:05:50 PM	0:00:35
4/4/2016 23:07:56 PM	0:01:38
4/4/2016 23:10:12 PM	0:00:34
4/4/2016 23:12:36 PM	0:00:29
4/4/2016 23:14:56 PM	0:00:34
4/4/2016 23:17:16 PM	0:00:30
4/4/2016 23:19:36 PM	0:00:29
4/4/2016 23:21:30 PM	0:00:30
4/4/2016 23:23:50 PM	0:00:26
4/4/2016 23:28:42 PM	0:00:41
4/4/2016 23:31:03 PM	0:00:40
4/4/2016 23:33:23 PM	0:00:39
4/4/2016 23:36:34 PM	0:00:38
4/4/2016 23:38:53 PM	0:00:20
4/4/2016 23:39:20 PM	0:00:40
4/4/2016 23:41:09 PM	0:01:45
4/4/2016 23:43:11 PM	0:00:32
4/4/2016 23:45:34 PM	0:00:30
4/4/2016 23:47:55 PM	0:00:30
---------------------------------------
4/4/2016 23:48:41 PM	Log End
Monitor Duration	0:52:00
Failure Summary:
	Count		24
	Total Downtime	0:15:19
	% Downtime	29.48
	Minimum Length	0:00:20
	Maximum Length	0:01:45
	Average Length	0:00:38
=======================================

Here's a recording I made, showing the strange bandwidth spikes that correspond to dropping connection: https://www.youtube.com/watch?v=lIXL6QD4uTk

Here's an example of the bandwidth monitoring on the second router. The spike perfectly lines up with when the internet goes out then comes back. It seems too periodic/perfect for it to be random noise.