It seems that your problem comes from the nut and locking-washer. I suggest you to use a special type of locking washer. in fact there are 2 brands. You may look at their web page for more details. They are called Nord-lock and Disc-lock. If you can't buy it, give me a e-mail. I may buy and send to you from here. Of course you have to pay. Ha Ha!
Abe Cheng
P.S. Please share the experience after you try the locking-washer.