Are you using a poly diff mount ?, if not then don't expect anything stable as you are being totally unrealistic due to the shit handling car the cossie is as std, if wanting ultimate stability get a 22mm rear arb, you simply won't believe the difference.
No doubt owing to the lower profile tyres they are following the road more accurately just the jelly on a plate wobbly chassis bushes can't cope with this.
On a private road I've done 150 on roads with repair patches etc on long sweeping bends and the car is one of the most stable cars I've had, with a 22mm rear arb and poly diff mount, it's horrible driving a car from the back and front at the same time, having to account for what the rear is going to do in relation to the front, the only certain thing is it won't be doing what it should be doing!!
tabetha