GPS Tuning problems

Hi, I have a tank rover using skid steering. With a lot of help from members of this forum I have got it working from my Radiolink transmitter and it responds to controls. I then set up a simple waypoint GPS run, basically along a road with a slight curve so three waypoints. The rover seems to start out oK. The gps is green and it goes forward about 10-12 ft then it suddenly swerves left off the road. Can anyone help with config suggestions and ways to test?