Have you done any diagnostics? When the problem is ongoing, are you saying it cranks good but won't start? If so, engine cranking, did you check for spark at the plugs and fuel injector pulse? If your missing both, I'd check the cam and crank sensors. Don't replace anything unless your testing points in that direction. How long since a tune-up? Any applicable trouble codes? Some of the national brand auto stores will check codes for free or get an obd2 code reader.