Could the settings for your gearing have gotten lost? You could try resetting it and reentering the data (yeccchh!).

My Flight Deck does something quirky. About every other time I ride, I get home and try to look at my average speed. The display reads "Err". The data needed to calculate average speed is there (elapsed time, distance) so I can do the math, but it is still annoying. I have to take my bike in for a 30-ride tuning soon and will have the LBS check it then. Shimano does not have any kind of direct support available, so you must work through the LBS.

I have seen my current speed display wacky values every once in a while. I got home one time and looked at my maximum speed and it read 79 mph, which is silly. I know for a fact that I never went over 65 on that ride (just kidding).

I love having the buttons on the brake hoods and virtual cadence enough to stick with the Flight Deck even though it does weird things sometimes.