The reason that happened is because EMI from the computer was interfering with the modem. happens quite a bit actually. I use to do technical support for a DSL ISP and i would suggest that to people who were having connection problems, and of course, they think that despite i did the job for 5 years, i had no clue what i was talking about. so, when i said do it or we just sit here until the universe collapses in on its self, they did it and look at that, it fixed the problem! ergh! people are so GD STUPID!
so check for that. also, insure you have filters on all your phones (if needed, not all DSL services require them) another thing, make sure the cable going to the phone jack to the modem is less than 6 feet. that can cause issues as well. if all else fails, try the modem in a different unfiltered phone jack if possible, because it could also be crappy wiring in the jack.