Arkie,
How is your GPS connected to your 998c Si unit? A direct connection is best.
What software version are you running in your 998c Si unit?
GPS does not know what direction you are moving until it can compare where you were at the last reading versus where you are now. This combined with the inaccuracy of the GPS system we are allowed to us makes slow speed direction and speed readings hard to accurately calculate.
We do have a new GPS Receiver that has a built-in compass: AS-GPS-HS. It will always know what direction the boat is heading, even at slow speeds, and make getting back to waypoints even easier.