Check the operation of the back brakes are they both similar.
Go out for a run of say 2 or 3 miles without using the brakes.
Be very careful here, are both brake discs the same temperature do not burn your self!!!!!!!!!!!!
I'm wondering about a brake caliper starting to sieze on which will be intermittent when it starts.
Also from speed if you just put the brakes on gently are you getting any judder through the pedal. In that case renew both front discs.
