It usually takes four things to spin - CG, elevator throw, rudder throw, and slow entry speed. I would check the GP web site to see if they made a mistake in the instuctions and have updated the CG first. Also, while it may not want to spin at the beginning of the tank, it might towards the end (which shows it to be a CG problem). But more rudder and elevator will almost always force a spin when the airplane slows down.

I'll bet it's warm there and your going flying.