When I have encountered a challenge like that, it is worth trying several things:
1) selectively disable plug-ins and see if that sorts the issue.
2) try with an alternative theme and see if it is theme-related
3) If you kept a fresh copy of the files, overwrite the entire wp-includes folder via FTP (no need for CPanel to do this) and start with a fresh copy.
Please also let us know if this has just started acting up or if this is a new install (and has been an issue from the initial install). What theme are you using? I was using Avada on GoDaddy and had to boost the memory allocation for better performance.