Start in Safe mode, his disables any custom apps to ensure the basic OS is fine
Secondly check the SD card > Ensure it is formatted as Portable
Check the amount of ,memory you have . Of the base 64Gb try and get 10Gb free, so clear the cache of at least all the apps you are having a problem with.
Note if you use an SD card as Internal all the content is encrypted, so if you have any personal data, images etc. and the phone fails at some point, which it will, then you will loose all that data, or in other words you will not be able to retrieve it with enormous cost.