High rpm cutout

What is your question?

1. In 'Park', if I rev my motor to 5,000 rpm it cuts out down to about 4600rpm then picks up back to 5,000. It repeats this about twice per second. 2. While driving and I hit the gas causing a downshift, the motor does the same thing (at a much lower rpm) No check engine light Timing belt has 100k on it. Any ideas on what this could be, or a diagnostic direction I should go in?