My 320CDI IL6 has developed a problem when the engine is hot and restarted after stopping for a few minutes. The engine shakes like a misfire, but after driving off and getting up to speed, it is okay and will idle fine. It seems worse when the outside temp is 30°+. It shows no faults and has had the mountings replace a year ago. I also get the common knocking noise at 1500 - 2000 revs when hot. I noticed the hot starting problem after filling up with supermarket fuel. Could this be the culprit? Where possible I use Total Excillium, which the car seems to like. Are there any other tests I can do to pinpoint the problem?