You dont possibly have a solid cam with hydraulic lifters ?You will have a abnormal vacuum readings and rough running conditions. Normally if one has a micrometer he can measure the lobe tapper.You should have around .002 difference measuring front to back.The taper along with the convex shape of the lifter is what makes it turn.If you havent pulled the cam yet I would steer in this direction.As bowtietim asked if this is the one breaking rockers maybe this is the case ?
