If that is occurring at idle speed or trolling motor speed, that is normal. The satellites are moving so that changes the distance from them to the GPS receiver. A more accurate antenna could lessen the effect, but I don't know if your unit can support a GR-50 antenna.
Your unit has an internal 16 channel GPS antenna.
If you have any radio frequency blocking structures near or above the unit, that could cause some of the speed changes also--it would "see" less satellites.