[Greenhornet2]

It's due to the amount of lift you are running high lift higher rpm weakens the springs faster. You start to lose spring pressure with a high lift cam, float a valve and it hits the piston. Stock ones should be changed too if the car is driven hard they wear out it just takes longer and if you start floating valves it might not be enough to hit the piston on stock stuff. My BTRs are dual spring they have higher spring pressure to be able to snap the valve closed faster than a stock would as the lifter falls off the mountain, the higher the mountain the faster the valve needs to close