I am experiencing severe misfiring after journeys of about 1 hour. The problem gradually gets worse and worse until I am forced to pull over and turn off the engine. If I wait for a minute and re-start it's ok for about 15 minutes and then starts to misfire again. The garage have looked at it 3 times but have not been able to fix it. Has anyone come across this and if so what did they do to fix it?