Carisma gtb set up question

[ChrisCorbett]

Can anyone offer me some set up tips for the carisma. Firstly when going over fairly low jumps (high speed) I notice that the back end keeps flipping out causing me to have to correct it losing a lot of time.
Also on high speed bends I seem to be getting understeer but on slow in-field sharp corners I get grip roll!
Any advice greatly appreciated!
Set up is stock shocks/springs and oil (never touched them), gear diffs 2 degree negative camber on rear, anti roll bar front and rear (1.2/1.1 respectively)