Bizarre readings on fuel computer mpg. Any ideas?

[Badgercote]

Returning home on motorway last night the trip fuel computer suddenly went doolally and instead of a fairly constant reading of 60+ mpg in 5th gear (and it was on a downhill section on a clear dry road) the clock started dancing up and down between 30 and 60 mpg. Never seen this happen before. It only seemed to clear itself when I dropped into 4th gear and the readings stabilised again.
Any ideas?

[RBH]

It'll be the usual Insignia gremlins.


The cars are well known for these type of antics.