WiFi problems and more

I've had my G3 for 18 months now and over the past 4 weeks I've had problems with the wifi dropping and reconnecting again. I tried the tips from people I found on other websites but nothing cured the problem. Usually I would forget the wifi and then reconnect to it. That would usually work for a few hours, but then it would drop again.

Around the same time my SD card would unmount too. I originally put that down to it being old, so I bought a new one, but again, same problem. I've not been using one since.

3 days ago the battery drained itself within a couple of hours and the phone failed to connect to any wifi at all.
In fact it kept resetting itself and I had to take the battery out so it could turn on again.
I purchased a new battery and the life has improved a bit but the wifi problems persist.

I did a factory reset hoping that would cure the problem, but nope, still not connecting to any wifi.

I was a sent here to seek advice from a Techie Forum, so hopefully you can help me out!
I've had a quick look and I feel a bit overwhelmed with all the root and flash talk....
Does this problem all come down to Lollipop being a pain in the ass?
Is rooting the only way to get around it?

Download from this link