Wouldn't know where or how to start, Ozzy. But if it is the A/C, as I'm sure it must be somehow, wouldn't the problem have been noticed and highlighted before now by a flyer on the forum? And if the control files of the plane were corrupt, how come it's a peach in the air. nnnrrrrr.
As you correctly say, a clean install has to be the first step. There is a copy on Ebay with five days to go, I am going to aim for that one, unless I find my original (have the box and paperwork - no disc, usual story) first.
Will update in a week or so.
Many thanks mate for your patience.