Some modems have been known to have a 'spiraling death' syndrome, where they are unable to 'jump up' to higher speeds when conditions improve. These speed negotiations may be transparent to the user of the modem. The user may simply notice that speeds become slower and slower as time goes by. Many manufacterer's ship these upgrades for free. Infrequent disconnects may reflect a temporary change in your connection.
Weather conditions can cause temporary changes in line conditions.
Excessive rain can cause increased line noise. Physical work, like construction or new line installation, can cause Pac Bell or GTW to temporary reroute line paths, perhaps through a noisier switch. Try calling a different location. Line quality differs from region to region, and it may be a problem with the lines or modem at the other end of a particular call. Try connecting the modem to a different wall jack.