does the wander coincide with a pull sensation through the steering wheel? or is it moving for no reason or with no sensation through the steering wheel, just constant correction to keep it straight?
If there is a bit raised road marking perpendicular to the direction of travel like a stop line, then the steering will twitch in my hands slightly. If I'm changing lanes and I cross over the lane marking thenI feel the car sway slightly as I go over the marking.
When I experience dissimilar road surfaces then the car will wander according to the road surface type.
On falt surfaces it is fine, you can't tell.
it was the same symptoms on the cheaper tyres I had and has not gone away with the Continentals.
my mv6 does the above, i will be checking the idler. 
hmm, cant really tell from your reply. Some things to concider,
does the car move or stear from the back? if bad enough it can be felt as the back moving around, its straight forward as that, but if more subtle as mine was at one point the car moves witn no pull sensation in your hands as you drive. This relates to rear subframe donut bushes ime. replace insuring the void holes point for and aft.
If the movement also can be felt through the steering by a pull then its a front end problem. note i have not experienced a failed idler and as yours is new then thats out along with the rest steering assembly from what you have replaced and checked, except for steering box adjustment. I adjusted a fraction of play out of min but it made no odds really.
Next is wishbones, knuckle and bushes. Ime play in these allows the wheel to move and as its attached to the steering the movement can be felt through the steering wheel as a pull. Get a bar and lever the bushes around, they will move ad they are rubber but there should be no play splits or tears. Steering knuckle to hub should have no play.
Set up, once your sure there are no faults with the car suspension or steering get it set up for camber at 1degree10mins, caster(less important) and toe(also on the rear only).
Trouble is poor set up slso makes the car pull through the steering ime as you need to steer against the pull to keep the car straight.
Tyres, myself and Kevin Wood have these and are happy with sc3 over all. However they are sensative to pressures, we both find they need to be between 34 and 31, 33psi i find ideal in uk temps.
Thing is with tyres if they have been exposed to poor set up i.e. excessive camber, then they will have worn excessively on the inside edge, this effectivily makes them geared or cone shaped. Roll a cone across a desk and it turns. This ware will poorly affect the tyre for the rest of its life on the car and you will feel the effect.
hth