Spontaneous reset of average mpg display.

The problem with my Octavia II, a face-lifted model bought new July 2009 and which has now covered about 5,500 miles, is that roughly every 400-500 miles the average mpg display resets itself spontaneously. The lowest mileage at which this has happened is about 380, and the highest is about 550. Last time it was 466 miles. The trip mpg display is not affected, and I don't use the instantaneous mpg display at all.

This happens without touching any of the buttons which control and/or reset the display, so it is obviously a fault. It could be either a software glitch (unlikely), an intermittent hardware failure, or it could be getting a reset signal from somewhere - possibly a faulty switch or wiring - that appears genuine to the computer, but which is clearly not a valid signal. The dealer has used the diagnostic computer on the system and can find no fault, and has no experience of this fault on any other cars. Consequently the dealer refuses to replace either the computer or any of the other components which may be causing the fault.

The first I know about it is when I glance down at the dashboard display and the mpg figures are varying wildly in accordance with the position of the accelerator, as one might expect when there is no long-term record of mileage for the computer to base it's calculations upon.

I sometimes (but not always) find that the unwanted reset has occured "in the background". Most of the time I use the average mpg display so that I can see in real-time if a spontaneous reset occurs. However, if I am using the trip display I do change occasionally to the average display to see how economically I am driving long-term. If the fault has occurred in the last few seconds then the average will have changed to an improbable (and usually a far too high) value. It is also showing a significant and expected variation in accordance with throttle usage over the short term. This variation becomes less as time progresses, as the mileage on which the computations are based steadily increases.

It happened recently when I was doing about 60mph on the M25. At the time I was coming up behind another vehicle and I was unable to pull out to pass, so I came off the throttle to slow down The fuel flow at that point would have been minimum. As the car could only have done a few hundred yards (if that) since this latest spontaneous reset, the displayed (and actual) mpg over that very short distance was improbably high. The traffic then cleared and I booted it to get past the car in front, and the displayed mpg went low, all as per expectations.

Has anyone else seen a similar fault on their car (such a fault may not be unique to an Octavia), or if I am lucky enough to have a Skoda technician read this, can you cast any light on what might be causing the fault?

All ideas gratefully received.