My current ISP is AT&T but I'm going to tell a similiar story first dealing with a different one. About 3 years ago my ISP was Comcast and their connection was fast at first but after a few weeks it started lagging at night and sometimes going off completely for several minutes before coming back on. After a while it would lag all of the time and sometimes the internet would just turn off and we'd need to call customer service for them to send someone out to fix it. No matter how many times they fixed it, though, it always turned back off and the cycle repeated. We switched to the slightly more expensive bellsouth and the internet worked fine for the next year or so. Then we moved to the current apartment and decided to try comcast again since it was cheaper. The very same night, however, the lag came back and it turned on and off just like it used to. So we switched back to Bellsouth (AT&T in georgia) and it's been running fine for almost a year but lately we've been experiencing the evening lag-spikes of comcast and now it's doing this all day. It's very frustrating as I play a lot of competitive shooters online and it's nearly impossible to play when every 3 seconds my ping shoots from around 45 to 215, sometimes higher, then going back down.
My router is a 2-Wire 2701HG-B. If you need anymore information just ask.