Why is my phone performing so horribly?

[Max Valor]

My device is an ls620, and the os is 4.4. Ever since last week my phone has been crashing horribly. At times it will constantly display a numerous amount of errors, forcing me to tap away 20+ errors sometimes. Apps force close constantly, it's super slow at times, and the phone restarts itself sometimes. I'm unable to access file manager or music at all, and I can't transfer files from it using a USB either. In addition to that, sometimes the screen will stay on even though I press the power button, I cant hear the other person in a call, and the keyboard randomly freezes. I've tried third party apps but they won't fix the problems I'm having. I tried deleting apps and checking for apps running in the background, factory resets, leaving the battery out of the phone and removing the SD card, but nothing works. This phone is an absolute nightmare to deal with right now, so I'd greatly appreciate some advice.