From what I have read when I was trying to pin down the issue; apps use a cell service to pin down a location quicker http://www.macworld.com/article/1159...ion_works.html
I think in my case the tree cover/tower switch/being in a small river valley cause the lose of coverage and subsequent loss of data. Here is a screen shot of a ride where this happened. (Always happens on this route with iPhone, upgraded to a Garmin 500 to avoid this)