Just thought i would give anyone with a similar problem a heads up... I took my c4 to Chevy dealer, and 4 of the best mechanics in town and of course got diffferent answres from each of them, from O2 sensors, Optispark, Injectors, Plugs, Plug Wires, Tps sensor, And fuel filter. Noone could tell me for s ure. Well i replaced the Tps ... no go. Next was the fuel filter which is in a shagnasty location so i took her to another shop to have the fuel filter changed. Told the mechanic what i was replacing the filter for. He replied " Your problem is the egr valve" At first i thought yea right but he said Just unplug the vacum line to it.. it will correct the problem. Sure enough it did. So next day i took him a new egr valve @ $45, a new egr solenoid @ $20, Paid him $65 labor and my c4 is like new. Long story short... if you are experiencing a stagger in acceleration off of idle, Unplug the vacum to your egr valve first. It is alot cheaper than anyone of the other alternatives.