iMac 27" Late 2009 freeze and boot stuck at 50%

Hi,

from some days my iMac sometimes freeze (only mouse can move) and, after an hard shut down, boot stuck at 50%.

iMac version: 27" late 2009

macOS High Sierra 10.13.1

It random freeze (also when I'm only using Firefox) and I can re-boot normally only after some PRAM resets or after entering in safe mode. After that I can use it normally whitout any visible problems, but randomly after some it happens again.

Any advices?

Thanks

Francesco