My car is idling very rough probable misfire gave code P0300

The problem presented suddenly, my father-in-law is an ex-mechanic so he thought it may have been the cat so when we got it home he told me to look under the car to see if it was glowing (it was after sundown) it was. So the next day we dropped the cat so it was out of the way still ran bad. Took it to the local parts store and had a friend who works there read the code from check engine light that was on it was code P0300 which is a random misfire. Replaced plugs, boots, and coil no change I am at a loss and so was our friend.