Troubleshooting LAN Sluggishness (need help identifying source of problem)

Over the past few weeks I've noticed internet sluggishness when browsing the web and lag spikes while playing League of Legends with ping spikes up to 3000 ms. I'm trying to figure out what exactly is causing this and would appreciate any help. I provided a description of my router and a typical ping result from a laptop over wifi to the router. Sometimes it's 1 or 2 ms while at others it'll spike to ~2000 ms as shown below.

• ISP: Comcast in southeast Michigan, which historically hasn't been an issue

• Router: Linksys WRT54G2 / GS2 running DD-WRT v24-sp2 (10/10/09) micro

Items typically connected to network:

1. 1 PC - Windows 8 (wifi with WPA Personal TKIP+AES - same for the others below)
2. 2 PCs - Windows 7 (wifi)
3. 1 PC - Vista (wifi)
4. 1 - Brother Printer (wifi)
5. 1 - Xbox 360 (ethernet)
6. 1 - Vonage (ethernet)

Pinging 192.168.1.1 (the gateway) with 32 bytes of data:

Reply from 192.168.1.1: bytes=32 time=1886ms TTL=64

Reply from 192.168.1.1: bytes=32 time=680ms TTL=64

Reply from 192.168.1.1: bytes=32 time=1ms TTL=64

Reply from 192.168.1.1: bytes=32 time=1ms TTL=64

What I've done so far:

• Ran MS Security Essentials and MalwareByte scans to make sure there isn't any malware on any of the computers.

• Used inSSIDer to check for interference from nearby routers, which there are at least 5 detected at any given time, but I've tried changing the channel to one without others on it and it doesn't seem to change the ping issue.

What else can I do :S? Should I run some sort of network monitoring tool to see if one of my housemates is running something usual on the network?