I've had AT&T DSL (adsl) for about 7 years, and just lately I've noticed my modem (Speedstream 4100) having difficulties.  Rather than all the lights being green like they usually are, the DSL one will change to red like it's trying to connect, and the activity light, which is usually always blinking, is sometimes off.  Could it be that my modem is going bad?  No. The red light means there is a problem with the line.
Ask you service provider to COME CHECK the lines. Often the phone company makes changes to phone lines and it might impact the DSL quality.
If the problem persists, consider a new modem.My family has had DSL for a really long time and we also had the ORIGINAL modem that was sent to us.  At one point, the internet kept disconnecting 3-4 times a day and we needed to restart the modem each time to get it to work again.  When we called DSL they told us that the problem was that the modem was just too old.  We bought a new DSL modem (Actiontec) and got it to work.  The first WEEK I would say it had similar disconnection problems.  However, those went away pretty fast and it's been SMOOTH sailing for the past 1-2 years that we've had the new modem now.  I don't really know if it was an error on the ISP's end that ended up getting fixed or if the new modem really was what led to the improvement.

Before getting a new modem, I would recommend calling your ISP and having them conduct line tests and maybe getting a technician to come out and check just to be sure.  If they find no problems but you're still losing internet every once in a while, invest in a new modem (after 7 years it's not a terrible idea) and see if it helps.

By the way, when the light changes to red, are you still able to surf the internet?