Hi, I am testing my APM and for some reason when I switch it into Loiter or RTL it will continuously drift south until it is on the edge of my comfort zone and I take control again (300ft maybe). 

The first circle of Loiter and RTL look good but the second is a little south and after 4 or 5 circles it way off and doesn't seem to be heading back.  It does seem to be able to "fly" the plane well (PID settings seem right) Stabilize and FBW-A work great.  It just seems to have a problem with navigation like the GPS points are moving south or something.

The wind was blowing south about 8 MPH but the plane didn't have any trouble flying up wind when told to manually and was making nice circles when in Loiter or RTL they just didn't keep centered on the right location or even return to the correct center once the got a long way south.

I am flying a basic Bixler with the APM mounted in a Bixler Chassis. I did have a camera on there too for some added weight but Stabilize and FBW-A didn't seem to mind the weight.  

I have noticed that when on the ground and hooked up to an XBee (my xbee has very very limited range) that the compass seems to be off.  It has the plane pointing in the almost opposite direction.  I have checked many times to make sure the APM (new purple v2 board) is pointing in the correct orientation in the plane.  Could the Magnets from the chassis be affecting the compass?  could an incorrect compass cause loiter to continuously drift south?

THANKS A LOT if you help!