Have you sorted out the crap geometry that is std on the 3 door/sapph ?
Get a set of adjusty rear arms or failingt that a set of shims from mike r(R&B), they do a LOT to help, correct the natural toe out of the rear, easy to fit and about £10.
What tyres are you using ?, you will without doubt need to poly the rear end, the beam bushes move about, as do arm bushes, the diff mount is a complete joke even when new.
You have a nice slop free front end being followed by a wild child rear end, it will feel as you describe, one of the bigegst drawbacks of the 3 door is the stupidly small rear roll bar, I fitted a 2wd sapphy one on mine(xr4i) after welding brackets on the arms, world of difference.
AVO do in my opinion use too stiff a spring for the road, they have a policy that if you are unhappy with the NEW coilovers, they will change the springs ONCE FOC for you, so worth ringing.
tabetha