RTL and loiter mode cause plane to constantly oscillates right and left

When I turn on RTL in arduplane my plane will start to turn in one direction and then it stops and turns in the other direction. It is never able to complete the turn, it just turns right, then left, then right etc etc. It’s almost as if it’s confused as to which way to turn. This also happens when I put the plane into loiter mode. According to the GPS I have 14-18 satellites. Does this mean that the plane does not have a GPS lock? This plane is a flying wing so it does not have a compass, only GPS. Also, FBWA and CRUISE modes worked fine.