mean02ss

I have QA1 double adjustable coil overs on my 02 SS and I have had a problem with the bolts staying put. The bolt used to connect the bracket on the bottom of the shock to the rear axel has failed me twice now. I noticed a noise and checked it out and turns out the bold had sheared off. The second time I was driving at about 55mph and the bolt either broke or came out (never found it in the road) and the suspension dropped. Any ideas as to why this is happening? I used a grade 8 bolt and a lock washer the last time I replaced it.