Take a look at these sites/pages. If I am converting things correctly, you are already running something stronger than a Grade 5 bolt. So are you using metric bolts? Wonder if you could try a Grade 5 in a standard thread--6mm and 1/4 are close, 8mm and 5/16, doubt you have anything bigger than that.
I'm just wondering if you got some crappy bolts. Dangerous game when you start substituting shear bolts with anything other than OEM--do so at your own risk.
https://www.boltdepot.com/fastener-i...ade-Chart.aspx
http://www.mcmaster.com/#hex-head-cap-screws/=10tbaeb
https://www.fastenal.com/en/76/metri...specifications