Shift lever won't engage

Last week, I went to the post office in my 08 Prius. When I tried to back out of my parking spot, the shift lever would not stay in reverse. It would stay long enough to give approx. one full tire rotation and would slip back to neutral. I repeated this process about 5 times in order to back completely out of my spot. I then put the car in drive and it did the same thing...went back to neutral. I then turned the car off, removed the key and started completely over. It did it again. It would not stay in drive for longer then a second without shifting back. After trying repeatedly, it finally stayed and has worked fine ever since. I called my local Toyota dealership and nobody in their service dept had ever heard of an occurence such as this. Any ideas? The only thing I have done differently since purchasing the car 2 years ago is replace the tires, which was 3 weeks ago. The engine seems to rev a little high upon acceleration since purchasing the tires, but I don't think that has anything to with the shift lever acting weird.