@ntutak: can you find out which version of OSPi you have? Version 1.0 and 1.1 use different switching regulators. It is possible that when the ambient temperature is very high the switching regulator’s internal circuitry might shut it down. To test whether the issue is caused by switching regulator, you can simply unplug the 3-pin wire, and power RPi separately using a USB cable. See if the problem continues.