I've had alot of stuff fixed lately and all was well for a little bit but now I'm getting a P0455 code (Evaporative Emission System Leak Detected (large leak) and I can't tell any difference in normal driving but if I really stomp on it or if I'm cruising down the interstate at 70 or 80 miles an hour there is a distinct hesitation or shudder. I know it's not the plugs or coils because they've both been replaced in the past week. I've got an APR stage 1 tune and I'm running NGK BKR7E's gapped at .028. I've had someone tell me it could be the EGR valve is stopped up or a vacuum leak. I've also ruled out a faulty gas cap. I've searched and searched but can't come up with anything. Any ideas? Thanks.