I don't think there is anything wrong with the FK05 you have, all cars will have a vastly different balance when the grip changes. Getting a different car isn't going to help you here, you just need a different setup for when the grip is high and when the grip is low.
Check out some setups here,
http://www.petitrc.com/reglages/xray...XRayT1FK05.htm
Generally big things to change are spring rates, oil weight and roll bar thickness. Another thing to change would be the roll centres, these can make a huge difference.
Hope that helps.