WHILE DRIVING ON FREEWAY VEHICLE ACCELERATED, CONTINUE TO GAIN SPEED. HAD TO BRAKE REAL HARD, VEHICLE CONT. TO ACCELERATE. EXITING FREEWAY AT HIGH SPEED, BLOW A STOP SIGN COULD NOT STOP. CONTINUE HARD BRAKING CAUSED BRAKES TO CATCH FIRE. FINALLY ABLE TO BRING VEHICLE TO A COMPLETE STOP. UNKNOWN WHAT CAUSED VEH. TO ACCELERATE