Signal boosters (amplifiers) usually cause more noise on the line which can result in disconnects. Also, it can kill the upstream signal (to the point where it is 'out of spec').
See also: »Cable Modems and Wiring Issues
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.