He is experiencing impact loads due to lash. The only way he wouldn't be would be if he were running zero lash.
There's no way of knowing what the cam lobes or lifters look like until he pulls them and does an inspection.
Just because it's not floating the valves like a rev limiter doesn't mean it's not losing control of the valvetrain. This loss of control is magnified on a cast cam core. The cam will literally start flaking off.
Capri, are you 100% sure on the spring specs? If so, I'd like to say a few words to your engine builder.
How high are you pulling the engine? 5500? 6500?