Why it keep rebooting?

Hi, i had used this device for such a long time. Almost three years and it ia going fine until I realize that it keeps hanging and became slower. But then, it become worse. When i try to format it and when it comes to connecting to wifi. It stop suddenly. And keeps rebooting over and over again at the formating process and suddenly it work again. Until just now, i restarting this phone again. But then it keep rebooting again and over again. When it finally work, i try to connect to wifi and it suddenly stop again. And keep repeating the reboot process. I cannot even try to connect to wifi. And for your information. My wifi has no problem because the other phone are connecting just fine. I cannot even open the wifi it will stop immediately so how can i want to cnnect to other wifi. Please help me. Anyone: '(. Please give me some tips to fix it.