That is an AT&T router. That type of variation is usually associated with a route which is thrashing a bit. In other words, it is overloaded.
If it is overloaded, then the it should change with the time of day. Prime time U.S. would be the worst, while early morning or late evening should be better.
Typically, if the problem persists, regardless of the time of day, then the router is having a problem other than the load. Could be something as simple as a bad cable, or an Ethernet switch which needs to be flushed of its MAC table.
