Mike,
I had that exact same problem last year!!!
It was discussed in the thread
GPS Problems 
In my case it was a case of Battery sizes and vibration when mounting the unit on the dash with the clamp - fixed with a small 3-4mm thick piece of foam inserted in the battery compartment.
I have since learnt I can rest the unit on top of the dash on a similar piece of foam in one aircraft I fly, which stops it from sliding about and absorbs any vibration.
Hope this helps,
Regards, SD..