hummingbird 797 gps speed varies

I have a Humminbird 797 SI.
When slow trolling (usually between .5 and 1 mile an hour), the speed readout on the screen is not stable.
It appears to recalculate every second and will bounce between .2 to 3 mph while the trolling at a constant speed.
The unit is using the internal GPS to calculate the speed.
My old 700 series unit, with external GPS, was very stable.
Hummingbird said the internal and external GPS should be the same.
Anyone else had this problem?
Any suggestions?

I've heard those 797s are NO good. So, just b/c you have to fish with my buddy I feel sorry for ya I'm gonna bite the bullet and make ya a sweet deal. I'll swap ya my 787 with the external gps. Speed is very stable! When ya wanna swap?