I can give you an exact answer to your problem as my friend has the same problem with his S60. The problem is with the front lower control arm bushes which are a great thick rubber items and oil filled! They flex like mad causing the tramlining. The police complained about this on their cars and Volvo fit a poly bushing but wont admit a problem to general public. The answer is to buy Powerflex bushes as my mate did and the car is now spot on. Please see this thread Simon
Master tech called today advising of test drive, found that car still at intermittent times would "slip in hands". They have ordered 4 new "bushes" and will fit tomorrow. Hopefully all will be rectified.