Battery problems on Samsung Galaxy Note 4

I have a Samsung Galaxy Note 4 which is about 2 years old.

Recently I have had issues with the battery. I will be using the phone and it will just switch off; when I switch it back on it says the battery has still got a reasonable amount of charge, 25% or more; today it switched off at 41% even.

I am wondering whether the battery has some memory but have read online that Li-Ion batteries don't suffer from this; I would have accepted the memory issue as the phone does go on a charging stand every night no matter the level of charge left in it.

So I am stumped as to what can be causing the problem. On occasions I can put it down to particular apps that are very resource intensive; high graphics games, use of graphics, location and mobile data, etc etc.

On an Android forum I found a routine for re-calibrating the battery meter but this says to allow the battery to reach 0% before full recharging; the problem I have with that is that it never goes that low before switching off.

I have also seen posts with instructions for clearing the cache using the three button power up (Power, Home and Volume buttons) method. I have done this before but if I try that again I just get a screen asking to confirm whether I want to install an alternative OS, to which I say No and it just reboots as normal.

Does anybody have any other suggestions?

Thanks
Rob H