What OS are you running?
I may be able to solve this mystery for you.
Sometimes you are given a lease on an IP, and that expires at a certain time during the day. Most times, you won't notice it because it is quick, but if it lags, you will lose connection. That is what happens to me sometimes at the same time every day.
Try renewing your IP before that time, I haven't had a problem with it for a long time.