I have a Garmin Forerunner 310XT that's been acting crazier than I recall it acting when I bought it. On a typical run, my Garmin will tell me I'm running, say, 8:40/mile then tell me a few seconds later that I'm running 7:40/mile. Then some random pace a few seconds later. Changing the setting from smart recording to every second doesn't make a difference. I'm not running in an area with any potential satellite obstruction, and for some reason, my mile splits always make sense.
Any ideas what's wrong? Is there a way i can use the fluctuations to figure out approximately how fast I'm going? Right now, my Garmin is worthless except during the half second it's giving me my mile split. And yes, I've tried resetting it and updating the software.