OTG,
That seriously stinks! Did you double check the settings for; age, weight, height, gender to make certain they are programmed correctly as Tri Girl suggested? I have the F4 which has worked brilliantly for me and is quite accurate. I just want to upgrade and have more options for my running. My dream is the RS400sd or RS800sd-neither are in my price range, but I can dream.
Hopefully you will get this straightened out soon.