The problem arises when you put said modem in a smallish room with about 800w of computing equipment and two people. On a warm day the room hit's between 25 and 27°C and the modem gets a little flakey.
Enter: the big fan!
I chose this one because it runs quite happily on 5v and is nice and quiet, screwed it onto the lid of a spare modem and swapped the lids over this morning. (hopefully) problem solved!