Possible Causes

WARNING: This page is about a different car, the 2003 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Loose timing chain or improper valve timing
PCM malfunction
Right side Variable Valve Timing (VVT) actuator is stuck in advance position
Spool valve in right side Oil Control Valve (OCV) is stuck in advance position