My wife's 98 Vanden Plas has suddenly become very difficult to start the first time each morning. It usually takes at least three tries, sputtering and missing until it finally catches and smoothes out. Then, after a few minutes of running, it runs normally and will start again without a problem the rest of the day until it sits all night again. I have installed new spark plugs, checked the ignition spark at each coil, cleaned the throttle body (including cleaning the electrical connectors), changed the fuel filter.....nothing has helped? It almost feels like my '82 XJ6 when I took the anti-return valve out of the fuel line (it stuck closed and the car would not run), which thereafter required a longer than normal time to start. Does the '98 XJ8 have such a valve, or is there some other reason for this behavior? Please help; any suggestions are welcome. Thanks.