I am using Lumia 520 since three years, two months back got blue screen error and formatted the OS and updated to new using online support now if i open some applications like Messaging, ShareIT and few other apps it was crashing automatically, other than that i can able to use whatsapp and all fine, what could be the reason i am not ready to restore factory because have to copy huge amount of data from the mobile.
Try doing a soft reset if you cannot afford a hard reset. Soft resets do not delete data from your phone, but they usually resolves most app crash issues.
Whilst your phone is on:
Press and hold the Volume Down and Power buttons at the same time until you feel a vibration (about 10–15 seconds). Ignore the "slide down to power off" screen.
Also, sometimes your SD Card might be the one causing the issues. Try using your phone without the SD Card and see if there is a difference.