You can try adding some winglets at the wingtips for more stability. Some say they work pretty well:

[link]http://www.rcuniverse.com/forum/fb.asp?m=10899566[/link] post #274
[link]http://www.rcuniverse.com/forum/fb.asp?m=11194764[/link] post #876, comments lower in post #883

I use the bubble version as seen in the secon link. They do improve stability but i built them rather large so the added drag draws an extra 200-300 miliAmps from the batteries during a P13 flight.