Remove the master cylinder and shorten the adjustable rod between it and the booster. My bet is the master cylinder has been replaced at one point. Often times you have to readjust that rod, because not all master cylinders measure the same.
I second this ^
I had the same symptoms on a truck, would drive around about 10 mins and brakes slowly locked up. If the piston inside is not fully released then the fluid can't go back into the reservoir as it warms up and expands, so the building pressure causes the pads to drag more causing more heat and thus more expansion.
