Water pressure gauge acting strange

Why does my water pressure gauge fluctuate at low speed. When I am going 5mph, it will read 10psi or so, then fall to zero, then back to 10. I can see air in the line going back and forth when this happens. At higher speeds, it is steady. Motor is a 07' 50hp 4stk and yamaha gauge. It is peeing just fine, just curious about this.

[boats.net]

Thermostats or pressure control valve may be opening, as long as you maintain pressure at high speeds it's ok.