This happens with my two computers, both having the hard disk has a lot of free space and was defragged, no viruses nor malware was found with two different anti-virus programs, lots of free RAM, however, one day it's fast, the next day videos from youtube or simple webpage browsing stammers and stops, nothing too serious, but annoying. Any knowledgeable answer to help me pinpoint where the problem is?.
Could be a few different things. The internet is only as fast as the slowest link. Youtube or one of it's intermediate hops can be congested or not working properly. If you have cable modem, your internet is shared with your neighbors. The more of them that are online steals bandwidth from you. Your ISP may throttle you after XXX number of bytes are transferred.
you may pay for 20MB, chances are on a good day with non of your neighbors using it you only get 15 or so, and maybe less depending on where the signal is coming from and what kind of medium it is coming through. most of the time unless you are real lucky it is coming across copper wires, rather than fiber. as the signal passes through the copper getting to the house/apartment the signal degrads, and you get only what is left of the signal. i cant recall if you said you were working off of a ethernet connection, or wireless, but if your wireless then youll only go as fast as your router, minus some for signal loss and interference. you may also notice it going faster until about 3-4pm.. thats when your neighbors get home and start using up some of the signal strength(like the previous poster said) the more they use, the more you lose. just remember, just cause you are paying for 20MB, the chances of you actually getting that speed at your house is slim to none. How slow is it going for you? if your paying for 20, and only getting 3-6MB(you can check on speakeasy.net). The amount of graphics on the website also can bog down the connection, or if there is music being played on that site.. just make the most of what you have and enjoy.
You can always do a tracert www.youtube.com and see how many hops it takes to get to youtube. Any one of those connections can be slowing it down. I think youtube uses flash, so make sure your flash is updated. Your ISP may also use burst so your connection will slow down after the first minute or two. Normally you don't notice it on small downloads, but it effects larger files and streaming video.