"When you say "all hell breaks loose" when it comes off, what does this mean. I wouldn't think the plane would fly that much differently, maybe just make a lot of noise..."
Yes alot of noise and if you are at high rpm / full throttle when it happens the whole side of the nose will break and crack out. It was fixable with thich ca and activator and flies fine now but it aint pretty anymore
As to the prop balancing, No, im not balancing them and this might indeed prevent this problem. You will find out i guess. Another guy at our field balanced his and like me his was fine for the first days 3-5 flights but on the second day he got a nasty vibration as well and his cross member had not come loose. Im thinking his prop is too thin and flexible. Ive gone to a graupner carbon composite CAMprop 11x6 now which is extremely thick and good results so far in the last 8 flights or so. Its still holding together.
About the epoxy on the cross member. You arent using epoxy to elapor on the cross member. Its plastic to plastic. There are two motor mounts of plastic and then a plastic cross member between them and this is what im suggesting to use epoxy on or better yet drill it out and use thick diameter carbon tube.