I agree the slip clutch is a likely problem. Also I would check the bushing in the flywheel. While the shear pin is not installed, grab the top of the flywheel and try to rock it in and out. If the bushing is excessively worn, it causes pins to shear also. I see you are showing a Case 830 tractor. I am not familiar with their normal operation. Is the pto able to be feathered on or does it slam on with a jerk? The heavy flywheel will shear the bolt if pto is not engaged smoothly.