Manifold bolts

[raptor2ride]

This past week I replaced my third manifold bolt. I have now busted 2 heads off the exhaust manifold on the drivers side. And one bolt on the passenger side. Both times its the bolt heads near the back of the motor, cabbin side. In both cases I have had to remove the header, clean the gasket, remove the bolt with vice grips and install new #10 grade bolts. After this the exhaust leak is fixed. I just wonder for how long or why it is happening in the first place?