2015 Toyota Prius V

My Prius V has slow acceleration and the RPMs seem to go unusually high on acceleration. This makes it dangerous to pull out from a stop sign, etc. in certain situations that would be fine in another car of the same make and model. And recently it has started to throttle up when taking my foot off the gas and even when pressing on the brake. This happens intermittently and randomly. Fortunately no accidents have occurred yet and I'm trying to get the problems resolved with Toyota, but the dealer says it will be hard for them to diagnose when the problem is not occurring and there are no warning lights. Less than 60,000 miles on it.

PRIUS V3 LUNGING BACKWARDS 1/21/2016 ON THURSDAY 1/21/2016, I WAS PARKED IN A BIG LOT AND AFTER MY APPOINTMENT, GOT IN THE CAR AND TURNED IT ON. BUT RATHER THAN PULL BACKWARDS OUT OF THE SPOT, I DECIDED TO FIRST MAKE A CALL USING BLUETOOTH. THE CALL TOOK ABOUT 10 MINUTES. I THEN HUNG UP, RELEASED THE EMERGENCY BRAKE, AND MOVED THE STICK SHIFT INTO REVERSE WITH MY FOOT, OF COURSE, ON THE REGULAR BRAKE. AS I RELEASED THAT BRAKE, THE CAR LUNGED HARD ABOUT 10 FEET BACKWARD ACROSS THE PARKING LOT "DRIVE", INTO WHERE OTHER CARS COULD HAVE BEEN PARKED BUT LUCKILY WEREN'T. I AUTOMATICALLY AND IMMEDIATELY PUT MY FOOT BACK ON THE BRAKE WHICH STOPPED THE CAR. NEEDLESS TO SAY, I WAS BEWILDERED AND SCARED BY WHAT HAD HAPPENED, COUNTING MYSELF VERY LUCKY THERE WAS NO OTHER CAR PARKED BEHIND ME. I DON'T KNOW WHAT CAUSED THE ENGINE'S LUNGE, I JUST KNOW THAT I HAVE DRIVEN TOYOTA PRIUS CARS FOR THE LAST 15 YEARS, LOVE THEM, AND NEVER HAD ANY DRIVING ISSUES. IN RESEARCHING THE PROBLEM ONLINE, I SAW THAT IT MIGHT HAVE BEEN THE CAR'S POWER TRAIN, SINCE SIMILAR LUNGES WERE DESCRIBED IN CARS A COUPLE OF YEARS AGO.