What's wrong with my LG g3(s)?

So I got an LG g3 for the first time a few weeks ago. Right from the get go, it seemed a little glitchy. Here are some of the things it was doing.

• freezing/lagging/unresponsive when apps opened, or when I tapped buttons..
• restarting itself several times
• turning itself off, once in the middle of a phone call
• restarting and hanging on the "opening" animation until I took the battery out
• freezing when I opened the stock camera app, or when I selected the video option
• unable to "find" my bluetooth speaker (1 year old) - that is showing up both on my macbook and my old samsung galaxy s4

So TODAY, I took it back to the store where I got it, and they exchanged it for me. Some of the same problems are happening.

This time....

• the stock gallery app is freezing every time I open it. I get the error message "Gallery isn't responding. Do you want to close it?"
• Lots of lagging.. then it will do 10 operations at once, get stuck on the "back" button, and open up its split-screen menu option and freeze. I've already had to reboot once or twice just to get it back on track. This also happened on the old phone.
• still can't find my bluetooth speaker

What is going on? It seems like such a great phone, in terms of design. And I've heard so many great things about them. I'm frustrated, because it's proven to be nothing but unreliable so far. Why is this happening on TWO phones?

Is there anything I can do to help it? Just know that I'm really not that tech savvy, and I can't just go into the system and make adjustments and know what I'm doing....