First thing that comes to mind is spark knock, that is why check the timing was suggested, without any info on the car it stabbing in the dark. I would start with a tune up, the usual stuff, plugs, wires, cap and rotor, check the timing and fuel pressure. Disconnect the battery while you are doing this and that will reset the computer and everything starts fresh, after driving it around and putting it through stop and go traffic along with some steady speed running then check for codes, there can be codes present without a check engine light.