[quote=seamstome]I have a 6600 and this is the worst thing about the machine. Mine "unthreads" about once a square when you use the thread cutter because I think it cuts the threds too short and they recoil when you take the piece out a certain way.[/quote
try to consistently use the Needle Up / Needle Down Button; this should really help with unthreading issues. I got this tip from the Janome 6500/6600 Yahoo Group.
if unlooping at #6 lever is an issue try threading your machine from the needle back for awhile. This also worked for me.
Good Luck!