XJ8 running rough

My 1999 xj8 is starting ok but when cold will not rev it also misfires, when warm it runs ok but lacks in power. It has new plugs also a new filter. When I change it to LPG it runs fine but when back on petrol its the same.I have checked the mass flow air sensor by unplugging it when runnig and the engine stops, thats ok.I have benn told that it mighrt be the O2 sensors on the cats. Ive also checked for leaks but all is ok. It smells quite rich from the back and mpg is all over the place.