Strange Connection Problems

[Ranger56]

If I go to http://www.speedtest.net and check my connection speed, it will often be the fastest it has ever been. If I test it 10 minutes later, it will be the slowest it has ever been.

This graph shows whats happening pretty well:
http://img220.imageshack.us/img220/8100/graphgj1.jpg

Does anybody have any ideas as to why this is happening?

This problem has been happening on and off for about a month, and we've already had to call Comcast once and buy a new router.

[kwhubby]

Although comcast boasts extremely fast speeds, often times there reliability is poor in certain areas. Where I am, ever since comcast came into this area there service has been terribly flaky, and poorly maintained. Sometimes with comcast particular nodes may have excessive amounts of people on it for the available bandwidth and make the drop of speeds and increase in latency at peak times a regular occurrence. Otherwise, line quality may be poor, and interference or line loss may be the cause to your intermittence.
This is all assuming your doing everything right, if your computer does not have any nasty viruses or spyware hogging your network, and if your network adaptor and router/modem aren’t faulty.
If you can assure that your computer or network isn't to blame, and you already have a new modem, you should get a comcast technician to run a test on your lines to see if the lines in the street / up to your house are ok. If the line problem is in your house or property, I don't know how willing they will be to help you or test the lines inside your house.
You might try a different speed test as well (find the closest location to your residence): http://www.dslreports.com/speedtest is a good link.
A quick and easy way to test your network is with ping.... After finding the default gateway ip address in your network, try pinging it in windows command prompt with
ping ip address /t /l 1000
If you see irregular latencies or any lost packets, your local network or computer may be to blame.