Are you sure it's your phone (hardware) and not the app that you are using that is giving you the GPS problems? Have you tried a different cycling app? If it is the phone, I would go with either of the two Garmins that were looking at. As Steve B mentioned, if you don't need turn-by-turn directions and some of the other bells and whistles of the 500, the 520 is a nice unit, from what I've been told.
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HCFR Cycling Team
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2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress