Are you torquing bolt to specs? Are the threads worn? Is there any type of lock washer or "spring washer" you could use with the bolt.
Here's a very good link to washers that might solve your problem. Lots of great information.
https://www.google.com/search?q=spec...hrome&ie=UTF-8
Read this:
https://vention.io/blogs/best-anti-v...-fasteners-178
Read about "Smart Bolts".
http://www.smartbolts.com/insights/l...nt%20loosening.
First thing I would do is try a brand new Grade 8 bolt.
What engine is in your car?