Battery issue? Or something else?

Battery drains which is expected since I’ve had it since 2017, I am going to replace the battery myself but it just started cutting in and out when I try to make a call or even just try to open up certain apps?? What I mean is I can dial the number it rings and I can hear it but the screens goes black so to choose an option on phone I have to hit power button to bring it back up and can’t even opens numbers to opt into flashes off n on. When I try to open up some of my apps it quickly flashes off and on and can’t open app??? What can cause this??? Thank you for your time. Sue c