I've had Comcast as my ISP for years as they are the only cable internet provider available to me. I subscribe to their 50 down/15 up internet tier, and have been receiving that reliably up until yesterday. It dropped to 5-15 up / 0.20 down. Also in games my ping randomly spikes to 1000+.
I'm using a Motorola SB6121 modem and a Netgear WNDR3700(N600) router. I power cycled both the modem and router, and when I ran the ethernet connection straight to my desktop from the modem, I was getting the usual 50/15 in speed tests. But when I connected everything back through the router, it was slowed again. Could the router be on it's last legs?
First try running some traffic through the router to a host in your network (large continuous ping or file transfer) to see how it copes; if this is okay, then try running My Traceroute (MTR) to a public server such as Google DNS for maybe 100 or so runs. If you suddenly see a jump in latency after the first hop, it would imply something is going wrong (queuing/processing/propagation delay) between the router and the ISP's gateway.
If all else fails, try connecting the PC directly to the modem. Then do a speedtest/pingtest. Eliminate all the variables till you figure out exactly what's causing issues. It very well could be something wrong with your ISP's gateway, but it's probably more likely to be a cheap router on its last leg. Consumer routers seem to break a lot...PFsense for the win XD
I recently called comcast and complained about the same thing. I live in WA, they had me power cycle my router then did some voodoo on their end and speeds are holding steady. Might be worth a try. Only using the router they gave me though. One of these days I'll buy my own...