As Jake said, the soft mount can be causing a lot of your issues. What I found is that when I originally designed my canister mount I only had a small amount of right thrust in the motor. I ended up needing more so I just shimmed the motor with out changing the canister mount. ANY SLIGHT pressure mixed with vibration will crack a rigid header. I ended up rebending my headers to line up perfectly with the canisters so there was no side to side or up down pressure what so ever. When I lined up my headers to bolt it in the place the holes were perfectly lined up and the gasket was flush all the way around. None of that, "when I tighten it up it'll pull it into place".

Are there flex header available from any suppliers for that motor/canister combo? That may help also but flex headers have been known to fail as well. I've got 500-600 flights on my flex headers (different plane than the one mentioned in my post above) with no issues but I also wrap them every year with a few layers of Teflon tape.

Hope you get it figured out.

.........Mark