I just gave the g a tune-up plugs,wires,cap,rotor,air filter & even a needed oil change. Before the tune, It was hesitating upon exceleration, it was sorta like a stumbling! Well after the tune up it stopped briefly, then started again right after! INFO: 139,000miles - 2 of the plugs are fouled with oil [i need a head gasket and more plugs plus I think this is the answer to what i'm asking]. When do I need to change the timing chain? and also how much are valve ajustments and are they hard to perform ie: skill level begin,inter,pro? thanx i know this shit is fucking long huh??????????