I just started having a performance issue that I am getting worried about. Last week while driving on the freeway, I stepped on the gas and when I hit about 4,000 RPM the engine started running rough and sounding strange like my timing was off or was having detonation problems. I do have a cold air intake from JLT and a SCT tune from Evolution Performance. Living in California I am running 91 octaine fuel. I called Fred at Evolution to ask what he thought was going on and he thought I may have bought a bad tank of gas. So I ran the tank down to nearly empty, added a octaine booster and filled it up at my local Chevron. Unfortunatly the problem has not gone away. In fact, it even runs rough at low RPM's now and performance is noticably off. Does anyone have a suggestion on how I should address this problem before I take it into the shop ? If I do take it in, I will most likely put the stock tune in and airbox on.