I know it's not much comfort, but I've had this too... intermittent loss of hill-holder function on a 2010 500 Sport, when the car was two years old. Back then, it was suggested that a dealer may have a software update that changes the 'hill angle' sensor (grade sensor) parameters, or at least recalibrates it. Perhaps, as well, try a search in this forum for "hill holder".
-Alex