I get code P0354 #4 cylinder after driving 30 to 45 minutes.
That code is for the ignition coil circuit. There are 8 coils, one for each cylinder. Temperature can affect the performance of a coil, which can function normally when the engine first starts, but malfunction when it gets hot. The coil will need to be tested, but most likely it will need to be replaced. This would also be a good time to check the spark plugs, if they have not been serviced lately. I recommend having a top-rated mechanic, such as one from YourMechanic, come out to the location of your XJ8 to run a diagnostic inspection, and replace your ignition coils, if necessary.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing