Can someone diagnose whats wrong w/ my connection?

It used to be fine a month ago, but ever since then it’s been mostly bad, and often unplayable.

It lags in 1 second intervals where all other players very consistently jump around and stuff. It’s as if their actions are being “updated” in 1 second intervals. This usually lasts for several minutes, and then it goes back to several minutes of being less severe but still noticeable, and then occasionally I’ll have very brief periods (like 30 seconds to a minute at most) where there’s no noticeable lag.

I checked my Xbox’s connection, and I have normal ping (75-100), 0% packet loss rate (occasionally I’ll see 1%), and my download speed fluctuates from 65 mbps, to 35, and to as low as 10 (paying for 100, thanks Time Warner…)

Is it this download speed fluctuation that’s causing it, or is it simply an indicator of something deeper? I don’t understand why that would cause me to lag so badly when it never goes lower than about 10, which is perfectly fine for online gaming.