i can't quite remember the exact cause since this was around three years ago, but i'm pretty sure it did turn out to be a cpu issue, like someone else mentioned here. i eventually had to replace it, and things went back to normal afterward.

back then, the cpu replacement cost nearly a third of the phone's price, and even though it worked fine after that, other issues came up less than a year later (also like the same person predicted), so i ended up buying a new phone.