Why would the bolt holding the sprocket to the cam twist off

I have a 2000 mazda protege 1.6 leader I bought the car it had been sitting for 6 months started first try I drove 10 miles the engine stalled . I hurd something was looseI tryed to start it nothing so i flat toed it about 1/4 of a mile I replaced cam sencer position sencer nothing took the valve cover off n seen the bolt was broke that holds the cam to the sproket. So I bought a used one put it in with the old bolts n races from my moter. Bought a inch lbs torque wrench torqued the cam in a three phase sequence to the torque specifications in the right sequence put it all back together changed the oil new timing belt new belts replaced the tenchiner pully kit both pullys andthe cam wouldn't turn so I loosened them from 125 inch pounds to 80 inch lbs n it turned like the other cam so I put it all together it started first try but the oil light is on. I take it around the block ran great went to take it home got about 8 miles the other bolt on the other cam shaft sheered off . Do I need a new head or is this a fluk