I had the same symptoms on my GS a few weeks ago, suspected sticking caliper on the rear passenger side. Took it to a friend who is a mechanic, he confirmed it and took the caliper off gave it a good clean and no problems since, only charged me £20, then told me I needed new discs and brake pads.. got them from Prolex UK £133,(with gold discount) and my mate fitted them for me, and cleaned the other caliper as well