i've had that sort of problem now and then. i think i sometimes just get bored playing in a relaxed way

Just realise that you don't NEED to use much effort to press the keys. you don't need to try hard to make the right sounds. be aware of this.
I just came back from a concert given by a classically trained (i think) pianist who plays a lot of jazz. his way of playing was very free, fluid and relaxed. it really appeared as though he didn't really need to think about what he was doing with his hands, and i think this way of playing the piano is typical among jazz pianists (although i'm not really too sure about that, because i haven't seen that many jazz pianists perform).
try to let go and relax emotionally/psychologically, be content with the sensation of relaxing. think less about HOW to make the sound, and be more aware of the sound itself (without mentally linking it to physical effort).
one other thing which could help, maybe - doing other forms of exercise away from the piano (strength, stretching, and relaxation exercises), to help work out any physical stress you might be feeling, and also to clear your mind a bit.
i don't know if any of that will help you.. that's just how i approach this problem