I was using the old DSL modem from 4 years ago, and I also had a new one (much smaller) SBC sent me last year. I decided to replace the modem this time (next would be the router). And bingo! The problem never happened again.
Maybe you can try it yourself. If you need a new modem, you can buy one from Ebay for about $15. Or if your contract is up, you can get a new one for free after rebate by discontinuing your service and signing up as a new customer. But the cost of shipping is about $15.