Just a guess but, I would say the gear isn't installed on the shaft in the correct location or the surface in the block, (at the bottom of the distributor hole), where the gear should ride is worn beyond spec.I broke a poly lock on the #2 cly,and this hick-up caused the timimg pin to shear.The cam gear did not move far just enough to cause the cars timing to be off and not run right,would not idle.This is the second time the pin has sheared,I think I did not damage the vavles this time.What is the fix????
EDIT: I misunderstood and thought you meant the distributor gear.
The cam bolt is not strong enough or it is not torqued correctly or
high valve spring pressure calls for "double pinning" the camshaft.