Here's my thought, and I'm not a brake expert so take it for what its worth. The fluid is getting super-heated somewhere. As the fluid gets hot it expands, creating a hard pedal feel. Next time you have this happen jump out and check each wheel for excessive heat. Yes, it will burn your hand so be careful! If you can't tell from the outside, reach under the car and get your hand NEAR the backing plate. One wheel dragging can cause this problem. If you bleed the brakes you'll probably also find the fluid from one wheel is darker the the others, indicating burned fluid. Keep us posted!