Why is my phone on crack?

Please help. Like an idiot, I left me Samsung S5 Active in the sun for about 10 minutes this past Sunday. This extended tanning experience seems to have fried it. When I realized it was overheated, the phone had shut off. When I powered it back on, it cycled through the Samsung intro onto the AT&T intro and then went black. After about 5 seconds of black, it would start this process again...Samsung...AT&T...Black. Wash, rinse, and repeat...and repeat... Once cooled, I tried it again with the same result. HOWEVER, after I connected it to the charger (battery showed a 91% charge and does continue to charge), the phone would reboot completely allowing me to make and receive calls, send and receive texts, etc...In other words, it functioned normally. The next day (after making and receiving calls and texts), when pressing a contact on the screen to make a call, the screen went blank again...and the whole original Wash-Rinse-Repeat (WRR) cycle began anew. When I plug it into a usb to charge, it reboots and operates fine for a period of time....then without warning, the WRR cycle starts. Do you have any idea what the issue might be? Please advise as I am not thrilled at the concept of using my Nokia (backup phone) for any length of time.