Shortly after filling up at a station I don't usually go to, the car started misfiring, off and on. Showed a code PO303. I changed plugs, fuel filter, dry gas, injector cleaner, and also swapped a different coil/Ign mod. All of the above made no difference. I then changed oil and filter and after a few miles it started running fine again. Around 50 miles later it started missing again. A chevy mechanic looked at it and said it was a stuck valve, and the 2.2 ecotecs are noted for that. I kinda suspected an exhaust valve anyway. Does anyone know of any good additives that will clean the guides/ stems, I don't think its bent because of the sporadic nature of it clearing up for a while.

Thanks in advance.