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.
How many miles are on the car. When you changed the spark plugs was there oil on the spark plug boots. If so you have valve covers leaking into spark plug holes. If not, the ignition coils sound like they are going bad and possibly should be replaced. When the car miss fires in the morning does the engine light flash off and on