Yes this is a common problem with the GPS system. Depends very much on your hardware, the features of the terrain around you and the position of the satellites above you. Unfortunately, GPS receivers cannot safily tell you when they have an unusable signal. The dilution parameter is quite different from hardware to hardware so you cannot just filter out anything above a dilution value of, say, 5.
Therefore, we recommend using devices with good receiving characteristics such as the HTC P3300 and stopping training recordings during pauses.