Unstable

[crossingover]

Hi guys
On my AU ive got 275/35/19inch tyres on rear and on the front ive got 235/35/19 reason being fronts are smaller is guard scrape.My tyres are fairly new ive got 4 shocks new short body and ultra low springs.My swaybars rubbers have all been replaced with noltec rubbers car is tight and very firm.
My problem is sometimes late at night i take it out and give it a squirt when im on the ring road turn offs and bends they are not sharp bends but windy ones,but what happens is my rear end will tend to let go half way through the turn when i say letting go i dont mean a full slide or so but as i put the car in to the bend turns in tight and starts to lightly slide through turn and exits excellent.
Could my tyres being different in size cause the front to dip in more and raise the rear?are 275 tyres to big for this car? At this stage im not sure?prior to my AU i had an vr clubsport with similar setup and it would hold road like glue.Im finding the au is not a good handler or am i wrong ?photos of height and rear rim width