Hi John . . .
I suggest taking the valve cover off and rotating the crank slowly. Examine closely the valves that are bending the the push rods (at max lift). It sounds like the valves are being lifted close to 'coil bind' . . . when things heat up the coils ARE binding and push rods bend.
There are a lot of variables here: cam lobe lift, deck machining (reducing block height) and head milling . . . not to mention shims under the valve springs (to adjust on seat height/pressure) . . .
regards,
stock49
