Phone powers down completely, then immediately restarts?

Hi-- this is my first time on here. I am 67 years old, not eligible for social security, and have no income. I bought a used iPhone 4s and found myself making multiple repairs, but that phone still has network connectivity problems. I am working up my courage to start micro-soldering (blue coil).

Anyway, this led me to try to start a business repairing and reselling iPhones. I'll be coming back to ask a lot of questions, but one at a time. On a particular phone, I replaced the digitizer and the back, and it worked! I sold it, but had not noticed that the replacement screen did not have a screen over the ear hole. The buyer wanted one, so she sent it back, I got a tiny screen and put it on, put the phone back together, and got a white screen (that question later). I replaced the digitizer again, and it worked fine, but by that time, the buyer wanted a refund, so I gave her one. I need to sell this phone again, but now it has spontaneously started--here we go-- finally-- to restart immediately after powering down, continuously, and this is causing the battery to run down in less than a day. Any advice?