When did this start?
A couple possibilities.
Spark Knock - Try Higher Octane Fuel
Timing needs attention - Check timing,
Also when was last tune up Cap, Rotor, Plugs and Wires.
And if haven't had timing chain changed it is probably time.
Another possibility is a weak Oil Pump, check oil pressure.
Wednesday, May 20th, 2009 AT 9:26 PM