Hi Peter Brown.

There is a thread on this issue here on RCU : http://www.rcuniverse.com/forum/m_5750756/tm.htm

They did as you have done and increased the vertical stabiliser surface and they also added area to the fins on the wing tips.

I am about to do this exact modification on mine.
Do you have a picture of how much area you added?
There are some pics on the thread above on how those guys solved the problem. Also some video of before and after.

I think this plane will always have this problem to a small degree at least. Very short coupled and nowhere to put a large practical amount of surface area. Without the plane looking silly anyway.