Make sure you are using the 300 Series GPS Antenna with the 6.01 Version Gauge. I have over 300 hours on one that it absolutely flawless. Also make sure your smart tow setting is on GPS with the speed filter turned off.
Remember that the system needs a quick "learn" if you add significant weight, change the prop, or change elevation - also if you have the DTS ignition switch, or push botton control, always turn the engine off by turning the key in the clockwise direction, trust me - or if you have the sweet control setup, just use the on/off botton on the control itself.
Hope that helps . . . . . .