If some others won't even sync at all, it means your line quality is poor and the modem has to work really hard getting data through.  So the amplifiers have to run at high power.  So your statement explains your overheating and crashing issues.  Suggest you try and place the modem vertical so you can get convection cooling rather than trapping heat under a horizontal circuit board.  That is what I like about the Linksys silver boxes, they have lots of ventilation holes top and bottom and are actually designed for mounting against a wall (i.e. vertical).