ok well
Comp Cams has pulled those lifters off the market. If it is an Indy block, was it drilled for Hyd oiling when ordered? IF not the bushings are failing due to a lack of pressurized oil. I have also seen the tie bars rub a lot on the ends of the blocks that can cause binding. With that much pushrod and spring pressure I would step it up to a 55mm or bigger cam. The cam could be deflecting under load causing some serious gyrations in the valve train. Rubbing pushrods and making sure that the pushrod has the correct ball on it to mate to the lifter cup.
Just a thought or two.
Todd