My iMac has gone bad

My iMac has gone bad. After a while of booting, it suddenly reboots and system shutdown repeatedly.

Why is this happening? I understand it myself. This iMac is of a certain age. It's iMac Ratina 5k late 2015. She is a great contributor to me.

She almost broke down many times. But she is a heroine who crawled back up each time.

Her internal HDD is already broken and was running on an external SSD. I tried several methods to revive her.

By a simple method, PRAM clear (rebooting and pushing command + option + P + R keys), She was revived to perfection again. I was relieved.

Soon a new MacBookPro will arrive. I eagerly await it. Until then, I hope iMac will hang on for a while longer.