2007 Prius Won't Stay In Gear (Reverts back to N)

Hey everyone,

This is my first post, my first car, and it's first issue. I really love my prius - but this was kinda scary.

Got this car used at ~59K. I put ~3K on it so far.

Arriving home after the car was sitting in the hottest temps it has experienced so far (it is from Nor Cal -> Utah, and we are finally getting our summer heat.) I stopped and attempted to park. What ever position I shifted to (R,D,B) it would immediately revert to N. I could hold the shift lever in position and maintain the gear, and so was able to park.

Later that night, after things had cooled down, I tried again, and was able to shift and maintain gears no problem.

Looking over this site I have seen this issue posted elsewhere but no verification from the poster as to results of advice taken. This issue worries me as shifting unexpectedly to N would be pretty dangerous in some situations.

I have heard about possible causes as...

-low voltage coming from the 12V battery.
-bad break switch.
-bad shift sensor.
-replacing the gear shift assembly box.

Anyone out there have this issue and feel like they really have resolved it/ found the cause?

Really appreciate any insight. thanks so much!!