a recent firmware updates to at least some motorola STBs has broken channel changing in certain situations. if you are suddenly experiencing issues changing channels on your motorola STB, try forcing channel changer 7 (-f 7) and see if you get any love from that.
more info can be found here: http://ubuntuforums.org/showthread.php?p=6350515
note that this issue does *not* affect mythprime *unless you are priming on a 3-digit channel*.