If it was an aftermarket camshaft it was most likely ground on a smaller base circle and required you use longer push rods. If you used the longer push rods on the stock cam you have the valve train running out of whack. You must have the right length push rods to keep it all running correctly. Stock cam equals stock pushrods. If you ran the stock push rods with a smaller base circle cam you may have damaged the lifters, rocker arms or valve stems.