Last fall, when temperatures were warm in the afternoon and cool in the evening I noticed drastic changes in the quality of service, depending on the time of day. The problem only got worse as time passed. Eventually, the service quality got so bad late at night (2am - 5am) that the modem could not maintain a constant connection. However, during the day the quality of service was fine. This made things very difficult when dealing with the phone company because they did not find problems during the day. The problem went on for a few weeks and they made several trips to my house and each time they thought they solved the problem... and they were wrong.
The problem turned out to be a combination of things. First, some of the wires in a telephone station/box down the road had corrosion on some of the wires/connections that were related to my phone-lines connection. This corrosion was not obvious at first glance, which is why the first few visits missed this. However the guy that did finally notice this said the wires were so bad he was surprised we had any connection.
Next there was also a small problem with another telephone box further up the road. This may not have been a contributing factor, but they noticed it when they didn't immediately spot the corrosion problem on the closer box.
Next moisture played a part in it. When the temperatures started to drop, condensation started forming in a certain spot and caused a bit of a short. Heavy rain also caused this problem.
I was also told by somebody that the expansion/contraction of the lines due to temperature changes may have played a part in it as well... although I am not sure about that.
Here are a few suggestions, if your dsl modem lets you monitor things like signal noise and line attenuation, keep an eye on it and log it. If your modem doesn't, consider getting a cheap one with those features from a local store that has a 30-day return policy.... You can use it and then return it. (just be cautious in using the software provided with cheap modems... it can destroy your computer's networking settings beyond repair.) I bought a cheap dsl modem that had those features and wrote a quick program the periodically read that data from the modem's web page. This allowed me to have a log that extended all through the night to prove that this problem occurred regularly on a pattern. Also, be sure that the other lines in your house are not causing interference. The phone company frequently tried to say the problem was not on their end and it was in my home. I disconnected all lines inside my house except the one that connected to the jack with the dsl modem when running my logging program. When they phone company tested that line inside my house, they couldn't find any problems. (if they did find a problem, they would have charged me for their visit).