iPhone 6 Battery Help/Advice

[jwc6160]

So my phone is doing the 98% to 70, then, 30 and the when I plug it in it jumps back up to 70. This happened a while back and it magically corrected itself. I called and they tested my battery over the phone and said it was above the line. A week or so later I took it in with the same result. The genius advised me to restore as a new phone(after trying to sell me the upgrade program over and over until I finally told him to stop) which was a huge pain and didn't help. I know others have experienced this so I'm looking for advice. I'd like to keep the phone for another year or so, but it also needs to work more than a couple hours before it shuts down. Will a new battery fix this even though I am "above the line" according to their test?