I just remembered something else that may contribute to inaccurate readings. I don't have the speed/cadence sensor because I have too many bikes for it to be cost effective to outfit every one of them. Besides, I'm not sure how well a wired sensor would work on bikes that fold up like my Dahon and Bike Friday.

Without the speed/cadence sensor, the Garmin is totally dependent on satellite signals for ride data (with the possible exception of the barometric altimeter). If there is any kind of interference like what you get when riding through heavily forested areas, you will get inaccurate readings.