MPG fluctuating hugely every few seconds

[Shaggy]

Hi there,
I have a Passat cc 2011. Under mpg on the computer, it was working perfectly until recently, now the mpg reading continuously jumps around from 10mpg to 200 mpg and to various different readings in between, all in the space of a few seconds. The reading then blanks out (as if resetting) before starting to do the same again. Any ideas as to what the problem is? Thanks for your help