Fortigate memory management

Hi, I have a Fortigate version 5.2.13 and from time to time the memory raise to 65 / 70 % and remains there. I need to reboot the Firewall in order to comeback to a more normal memory of 30%. It seems that when the memory goes up, then it is not able to free the memory correctly. 

I'm planning to upgrade to a newer version. Any recomendation? 5.4 vs 5.6?

Thanks