Thank you to the folks who have responded to my enquiry.
I was flying an A320 from Birmingham to Frankfurt and the problem occurred over Belgium. The weather was set at the default setting.
I have cleaned out the case, though it was already in good condition.
Another flight has been carried out with the following result :-
A320 from Chicago Midway to Minneapolis, with the sides of the case not fitted.
After 40 minutes of flying, the picture on the main monitor faded [ but not completely] and everything froze. Incidentally, the second monitor showing
ATC and Sat Nav froze but at no time did it fade. The notice on the main monitor said "Microsoft flight simulator X [not responding]"
After 8 minutes the flight became operational again for 12 minutes before freezing again. 10 minutes later it continued and I landed at KMSP
after a further 20 minutes.
Regarding the comments of Quickmarch, I finished up looking at four boxed graphs in line. The first one had 25% to the left of it, and the trace seemed to
confirm this. The second and fourth graphs were fairly active and went from top to bottom of the scale, and the third ranged up to 75% of height.
Beneath all this the Physical Memory flat-lined at 2.02 Gb.
The Physical Memory table read as follows :- Total[Mb]--- 8150, Cached--- 3908, Available--- 6031, Free--- 2266.
I hope that this is of use.