Internet lag spikes, need help please.?

I have cable internet, and about a month ago I have started to have giant lag spikes, for a few seconds everything will be fast (700kb/s download) and then for the next few seconds it'll stay really slow (5-15kb/s download).
I plugged a computer directly into the modem and the lag stopped, the modem is brand new.
So I bought a new router, and behold, the problem is still there.
So it's not the router, its not the modem, other people accross the street don't seem to have a problem with my ISP.
I have 9 computers accessing this internet, but not all are on at the same time. This problem exists even with 1 or 2 plugged in as long as its through a router it seems.
Not all computers have antivirus/the same antivirus, we use mac osx, windows xp and vista.
What is going on here, I am completely baffled.
 
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