I had this same problem with a .40 size sport plane. It turned out to be a mixture of thrust loading, and rudder issues.. A high angle of attack would wash out a bit of the vertical stabilizer, and cause the plane to skid pretty easily in that configuration. I just got pretty good at adding a bit of rudder when entering verticals. Like the previous post said. If you have a computer radio, you could perhaps mix out the flaw.

FW