I have a 99 xc70 168,000. I drove from ohio to key west, no problems. On the way back I got a code p0304 #4 misfire. I cleared the code, switched coils, and still got 304. I drove some more, checked it again and the code was 0300 multiple misfire. I got home, put new volvo plugs in. I cleared the code and ran it. It took a few minutes but 0304 came up again. a volvo shop said check signal to coil and computer might be holding the injector open and flooding the cylinder. Please help.