Well i always use Megui to reencode the files to x264 then use Tsmuxer to create the AVCHD.
There's a guide for Megui here:
Plus a good forum here that also has some great presets for Megui
http://forum.doom9.org/forumdisplay.php?s=bb3ee9b98ce331ab358093bd749d1ca4&f=78