I have a ‘97 Chrysler Concorde with the 3

I have a ‘97 Chrysler Concorde with the 3.5 6 cylinder engine. Recently I had a gas leak in the engine compartment. It needed a new fuel injector for cylinder 4. It was repaired on Feb.6th to the tune of $244. I picked it up the next morning (Feb 7th) and drove it home (5 miles) with no problems. On the morning of Feb. 8th, while driving to a hospital, it started idling very rough, check engine light came on solid, and periodically it would flash on and off, and throwing code P0304 and P0306. I took it back to the mechanic on the morning of Monday Feb. 11th. He read the same codes on his reader as I had on mine. He took it all apart, said it was a loose O-ring. He said he fixed it and He put it back together - no problem. I headed for home and hadn’t gone one block before the check engine light came on doing the same solid and flashing thing. I quickly turned around and took it back to the mechanic. He read the codes and said it needed new plugs, wires, and coil pack to the tune of $233.00. I drove it home and just as I was pulling into the driveway, the check engine light came on doing the same solid and flashing thing again. Immediately, I drove it back to the mechanic. He read the codes again and it was throwing P0300, P0304, and P0306. He said that, since the car’s computer was old, that it might take a long time to reset itself. He said to drive it for 2-3 days, if it didn’t clear up, bring it back and they would take another look at it. I haven’t taken it back yet. Yesterday, Feb.19th, I read the codes again myself and it was still throwing 0300, 0304, an d 0306. Could you tell me if there is somewhere we should look for the likely problem or how to diagnose it, etc?