fresh tyres? I have noticed an increase in tram lining with the new falkins on the rear.
Dont want to say i told you so, but....
Worst tyres i have ever encountered for tramlining and directional stability on the omega, appalling, esp the 912s, ask Kevin. Jamie also drove my car with them fitted at the Ashridge meet and i quote"horrible....horrible car, awfull, you just dont know what its going to do next" constant correction needed to keep the car straight. He was a bit(alot) more confident at Newent, and was suprised i think, that all i had changed on the car was the tyres, to sports contact 3s.
I likened it to driving on 4 water balloons. 452s or what ever they are called, are less affected, but i still spent most of the life of my old 2.5 with those fitted, replacing suspension parts to no avail. Couldn't beleive tyres could cause such symptoms in a car.
Only thing i will say for them is they are very quiet.
To the op, if you have good tyres fitted, not cheapo crap, or worse, falkens, and they are not waring unevenly then your probably looking at failed wishbone bushes. Lemforder (£110 ish) make the originals for vx who just add a gm sticker and double the price. Also check stearing idler, track rod ends, springs with the ends snapped off, and anything else while your there then get the car set up. Full Geometic for camber caster and toe.
Hth