Speed Issue With An Unusual Fix - Need To Know Why

[setotitan]

ISP: Brighthouse (i.e. Time Warner)
Location: Lakeland, FL

I'm a pretty tech savvy guy, in fact I'm in web development, however I don't understand what is happening here. About once a week sometimes sooner my internet speed will drop from about 20mB to 3mB. This is particularly frustrating as I work from home and I'm constantly downloading large files from clients and co-workers, hence why I got the faster package. I'm directly plugged into my router via ethernet cable. Now when I just pull the power cord on the router wait 10-20 seconds and reconnect there's no change in my speed. However if I unplug it and wait 15-20 minutes and reconnect then my speed is back. My guess is because I'm obtaining a new IP or a new connection when I leave it out that long. Where as if it's only a couple minutes or seconds it holds the connection waiting for you to plug back in.

Now my question is what is happening and why do I have to do this? I've contacted my ISP and they say everything checks out, they don't know what's going on, and they can replace my hardware. However obviously it's not my hardware given the facts. It's something to do with their connection. My personal theory is that they account everyone X amount of bandwidth and if someone is using an excessive amount there's some protocol to slowly ratchet it down to a lower level. Obviously I can't prove this and it sounds more like a conspiracy theory, which is why I'm out here hoping someone can help me with some answers. Thanks!