It is not supposed to behave like that.
- A failing or too slow SD memory card can spoil the performance and make the whole phone unreliable. Backup all personal files and replace with a reputable brand SD from a reputable seller. UHS-1 speed class U3 is recommended.
If it is like you describe without SD card, then I suggest let a technician or a skilled friend inspect the phone. - It's easier to evaluate properly hands-on .
I hope you get it sorted out.