The two big reason of flickering monitor [solution]

It can be ram issue and other wired  connections issue (inside monitor)

1) For the ram issue:We can easily remove the ram and clean it with soft cotton that Dont damage and then put the ram into the slot but, in many as well as my case is (critical memory died) which is ram corruption issue I changed the ram and its works for me

2) For the wire inside the monitor:
We need to open the monitor

(here is how to open the monitor video)

after open check wire if disconnected or half connected then remove it fully and reconnect that pin into slot this can be also the solution

Note:  If this not work than may be it's a software issue or HDD issue, So for HDD error diagonse remove it from motherboard and change the SATA0 to SATA1 input (it is a female socket) and for Software issue, this can't be diagonse easily so let just reinstall the OS.