DSL: why does my internet slows down, then gets back to normal speed?

its keeps doing the same thing over and over again... slow, fast, slow, fast.... 5- 20 minute period......





the slow speed gets down to 139.8 kbits/ sec according to verizon's speed test.





im on a router, but nobody is nobody using the internet, router shows no activity on other computers.|||that happens all the time. During peak hours, everyone is using the internet, which slows everything down.|||Could be the phone line cord or a virus. Do a test to see what the problem is then call Verizon.