Weird battery drain - iPhone7

I've had my iPhone 7 about 2.5 years and the battery life has remained pretty good generally. However, lately I've noticed something odd going on.

The phone will behave pretty normally but sometimes when it gets down to around 70% or less it's like a gremlin gets in it - something starts really draining the battery and it'll count down 10% or more within a minute. Sometimes killing the app I'm using stops it (not always the same app- most often Safari or Facebook tho, and once it happened while I was updating apps) and sometimes rebooting the phone will stop it.

So for example, today it was on 71%, after around an hour and a half of use, when it started to count quickly down and went down to 58% in 20 secs until I rebooted it. After that it stuck on 58% for another 15 mins or so before dropping to 55% suddenly then starting to count down again, until it stopped 30 secs later on 51%. I plugged it in for a short while until it was back up to 60 and since then the drain has been perfectly normal, around 6% over 20-mins use. Any ideas? Is it just time for a new battery or a new phone?!