Wandering Minstrel

I have fitted some new tyres to the back that have a higher ply rating and the pressures are up too, but now it seems that the front end is wandering. Before the new tyres I had Michelins all round with softer sidewalls, I would have thought if it was going to wander it would have done it then. The fronts are still the Michies but it just seems to wander across the lane. I havent done a high speed run yet, but am a little reticent to do so.
Any ideas?