If you are downloading from the same site when you are experiencing the decrease, then it could be on their side, but it seems more likely to be yours. A very likely culprit is your ISP throttling long downloads for "QoS" or crappy DSL/cable service causing packet loss.
for wireless you are probably getting interference. Change the channel of your wifi, and be using at least 802.11g, definitely not 802.11b. (requires the wireless node to support 802.11g, which pretty much all do nowadays) 802.11n if you can. Also: http://xkcd.com/654/
mostly sounds like a crappy ISP to me, though.
The channel change didn't work.
We switched out the cable modem for a new one. This looks to be the solution sine the new computer has had zero internet problems. Unfortunately, it looks like I need to find a new driver for the old computer since it will log into the internet for a minute, then disconnect.