when i worked in a garage in philadelphia, a chrysler lebaron came in with a similar problem, would drive fine for a while then the brakes would jam on leave it sit and cool and it was fine, till it heated up again.. .. when i checked the resevoir it looked as if there had been oil poured into it (instead of brake fluid ) so we changed the master cylinder and flushed the brake system, solved the problem.. maybe not whats wrong with yours but theres a wee story for ya anyway..
