Where are you seeing that it is spiking exactly? (make/model of equipment is helpful).
What (and how many) PC's/devices are on your LAN?
In brief, if could be anything from PC's with a game service client (like Steam) which are uploading game data to other players, or similar service. It could be a program/windows rigs downloading updates all at once (more likely Steam or similar).
It could also very well be you are under a network attack. You need to look at your router and/or cablemodem/DSL modem log files and search for key phrases like "DOS" "attack" "scan", and see if anything is coming up. Another good sign of this is not only large lag, but actual loss of internet temporarily.
Then again, it could just be your wifi router/access point is tucked away far from where you're surfing on your wifi device and interference is causing a poor connection. Again, this is why the details are literally everything when asking for assistance
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