You mention you use High Tensile bolts...and I dont mean the ones in the grab paks you buy from Repco, Autopro and the like..
I assume you mean 4.6 or 8.8 black, this number will be cast raised on the bolt head
The stress and load force required to sheer either one of these bolts is.... ENORMOUS....
Buy either of the above, prefer the 8.8 , some flat faced hardened washers, check the OD of your mount holes for the correct sizing, ( they may be worn or oval with so many failures, if so re drill and go oversize) use Nyloc nuts and thread lock, you should have a minimum of three threads shy of the nut and torque to 59 to 78 NM and see how you go...
NB: If you have any problems getting the bolts, drop me a PM