A phantom has taken over my iPhone 6

A phantom has taken over my iPhone 6, iOS 11.2.6 — Randomly it makes phone calls, takes pictures, repeats previous actions I have taken, like opening contacts, searching for a name, selecting a name, and so on. It scrolls through old internet searches. Even when the phone was turned off, it just started playing a song that I downloaded from youtube a few years ago. Another issue I am experiencing is that sometimes when I am trying to add an event to my calendar or typing a text msg there is a lag time from when I touch the key and when the letter shows up, or it types the wrong letter, or it just completely freezes up. I have experienced similar issues like this before but never to this extent. I also noticed that when the issues start and I have gone through the obvious steps to fix it like turning the phone off, then back on, closing the app, then reopening it, do a complete shut down of the phone, like when the  appears on the screen. I check to see if there are any app updates pending and I will download the up dates. That usually takes care of it, but sometimes it doesn't. In those cases I have noticed in the next day or two there will be a bunch of updates that show up, then once I complete those up dates things are running smooth again. But this has been going on for weeks, it may work ok for a day but then it starts acting up again. Using my computer I contacted apple support, after 45min of texting back and forth and trying different things, he had someone from apple support call me. During this support call we went through a menagerie of things to try and fix it, but nothing worked. She recommended that I head to the Apple store to speak with someone from the Genius Bar. The Genius Bar dude hooked my phone up to some diagnostic thing and said he thinks it's my battery and that it needs to be replaced, although my battery was 93% charged. I agreed to a new battery, he placed the order, but I have not heard from them yet. Please I have had enough, and would appreciate any advice you have that will help to rectify these issues.