Problems with my S5380i

Beforehand, if this is the wrong sub-forum for asking questions, please move my thread there.
Thank you

Hello everyone.

This has been a long coming thread that I postponed for a long time. My phone is pretty old (6+ years minimum) but I feel I kept it in very good condition until recently (about a year ago or so) where various problems started to happen.

I installed a custom rom (Razodroid 2.5) from here and it was really good for a while. But then stuff started happening and I thought it was the rom, so I changed it to Retribution Extreme Barebones, which didn't fix any of my problems so now I'm writing here in hopes that some of you may have the answers I need.

1. Slow response / getting stuck
   Phone is very slow now. Sometimes it gets stuck and I have to wait 5+ second for any command to pass through (like swiping the screen, typing into a search bar, opening apps etc). The phone isn't very strong on speed, but it wasn't like this before. I don't have any major apps installed on it, nor did I have before. Basically the largest apps are Google Play Services and Youtube, along with some minor apps that I put on sd card. When I call someone or someone calls me, the phone can get stuck and turn off display and I can't get the display back on for about 5+ seconds.
   


2. Battery bug (instant shutdown)
   This is my biggest problem. Whenever I am doing something that takes the whole screen (browsing images, watching videos, playing a game) the phone has a random chance when being below 60% battery to INSTANTLY SHUTDOWN and is telling me that the battery is at 1%. When I turn it back on, the battery is anywhere from 50-20%. It doesn't make any sense. If I want to watch something and I don't want this happening, I have to put it on a charger and keep it like that. I thought this was a problem of something with the custom roms, but I've changed them and the problem is still happening. I'd appreciate any help on this

I hope the details I've provided are good enough. If you want to know more, please let me know.
Also if you need logs from me, let me know. I appreciate any help you can give me, along with suggestions. Thank you :)