I have a simple question, just don't know the answer.

[Mr 5 0]

Here's the tech book answer:

Valve Float A high-rpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain.

The simple answer is that your valve springs probably need to be replaced.