After some electrical upgrade work recently, I got car back and instead of the Average MPG refreshing every 100 or 1000 miles or whatever, it started refreshing every second or so, crazy reading from 0.1 to 99.9,and no matter how often I reset, it kept doing the same. However, the longer I let it go, the less frequently it changed, and has now settled down to a reasonable average after around 200 miles driving, and only changes every few miles,and by 0.1 or 0.2 at a time. Not right, but getting there.
It has also been suggested that such faults may need the battery disconnecting for a few moments, you could try that,it may help.
How did you reset Avg MPG, and how long do you leave it on 99.9 before resetting again ? ( in case this information is helpfull in anyone else assisting !)