There must be something else going on I think, there is just no way you can shear off the bolts with the engine torque/momentum. The halfshaft CVJs see the same torque x gearratio (plus/minus maximum wheel torque even) and they are fine. Where there any parts made of aluminum? The bolts are critical, unless they provide (and can keep) the correct preload the CVJ will slip back and forth in the companinon flange and may shear off the bolts or they will work themself loose.
Steel flange, correct bolts (12.9) at the correct torque is a must, washers are critical too, must be made from hard enough steel (OEM stuff basically). Make sure the surfaces are clean too to get as high friction as you can, too much grease in the CVJ working it's way out in the bolt holes can be bad.
