How worn are your discs? If you've got a noticeable unworn ridge at the outer edge this can cause your problem.
If the disc thickness is within tolerance you can grind the ridge off. I do it by removing the wheel and supporting that corner of the car on an axle stand. I then lay the removed wheel in front of and in contact with the opposite front tyre (as a wheel chock) and firmly apply the handbrake. Making sure that the opposite wheel & tyre are firmly on the ground I then start the car and put it into a forward gear. The car's differential will then do its job: the wheel you're working on will spin merrily in reverse and the opposite wheel & tyre will remain undriven. You can then carefully grind down the ridge on the disc using a suitable tool (in my case it's a PermaGrit file or even a large and crappy old flat screwdriver!).
As long as you use common sense and ensure that a friend is in the driving seat ready to take action if anything goes wrong then it's a safe procedure. Don't forget to wear eye protection.