engine cranks over but does not fire ocassionally?

You most likely have a problem with your ignition system. These are some of the more likely reasons this problem can be occurring: 1) bad plugs an wires 2) faulty cam sensor 3)faulty crank sensor 4) faulty distributor

I have an 2006 2.8 it had the same problem I had it in the shop 4 time one time for a week the security system lincked to the key they shut the security off it fixed it later they put a new chip in for the key to start

KEY IS SLIGHTLY WORN OR IGNITION PINS INSIDE ARE WORN EASY FIX REPLACE IGNITION BUT IF YOU DO SO COMPUTER WILL HAVE TO BE REPROMT TO START BACK ABOUT $37 AND A 2HR FIX THEN GM PART $90