crazy data from EM-408 - where to begin trouble shooting?

I am using a GlobalSat EM-408 (on a sparkfun Geochron blue). I cannot explain the bizarre data I ma getting and do not know where to start looking for the problems. The device in in the cab of a piece of equipment and indicates that it does have a lock.

A typical log from a piece of mobile equipment show it constantly in motion for 3 days when sampled every 15 sec even though is is not used and stationary for 14 hours per day. Even considering the lack of precision/accuracy of GPS, it makes no sense. How can I begin to trouble shoot this?

Thanks!