If you guys are using the mount that places the GPS Receiver behind and below the top of the unit than you are depriving it of the signals from any satellite that may be in the direction of the unit. The unit is probably blocking some of the signals that couple help further define your position – thus improving accuracy and lessening the speed jumping around. For best results you should have the GPS Receiver mounted so it has an unobstructed 360 degree view of the sky from a 10 degree to horizontal angle up. For the mounts like it sounds you are using see if you can fashion some type of riser to place the GPS Receiver just about the height of the unit and let us know if that helps with the low speed readings.

Greg Walters at Humminbird