Ok its happening again my pc reboots as soon as open any application except terminal
Can you post the contents of the logs?
I do not see anything in those logs that relate to your issue. If anyone else does, please speak up.
You are using Intel Graphics, right?
Can you please relay the output of
sudo lshw -C video
Are you using a 4k monitor?
Also, are you logging in on Wayland?
Do you have the Night Light extension enabled? Or the wallpaper changer?
no.
Btw it's working fine now the only problem i'm having now is, it's a little slow and the monitor shuts down once in a while for 5 or 6 seconds and then turns on automatically.
I had a similar experience for a while... It took me a lot of time to figure out the cause in my case.
The cause turned out to be using an HDMI cable and a VGA cable on two different monitors. Once I switched out the cables, the problem vanished.
You might try checking your cable. Preferably, if you have access to a new cable to test.
If the cable is the culprit, then it having been bumped or jiggled may have stopped the flickering.
i checked the cable but the problem's still there. and my cable is not that old so i don't think the cable is the culprit
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In that case, let's check your logs. Please open /var/log/syslog either with Text editor or terminal and scroll through looking for any PCI Bus errors.
If you use the Text Editor, you can use the search feature ctrl+f to find terms based on search. Like ctrl+f enter in pci
EDIT: My slow brain remembered:
i915.enable_psr=0 grub parameter that you might also try...
sudo nano /etc/default/grub
Add the above parameter after quiet splash:
"quiet splash i915.enable_psr=0"
ctrl+x, y, enter`
sudo update-grub
there is "intel_iommu=igfx_of" after quiet splash. i don't remember why i added it. should i remove it
Yes.
This was a suggestion made earlier that did not help to resolve the issue. You should remove it.
done. it worked. thank you so much 
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I login to my pc after 2 days and now i'm having same problem again
maybe try i915.enable_rc6=0...
So, I tried it and it worked until now, it start happening again today
I have a question
Can power connector plug cause this kinda problem?
Yes, that is possible.
There are several possibilities.
I still think it is more likely that the Graphics are involved.
It may be that the integrated graphics are running out of memory after a while - as it uses your system RAM to have VRAM.
You could test that by freeing up RAM every several hours: