Mac book pro running forever spinning busy icon, mouse moves but no use, keep going to black screen

I had this question after viewing MAC frozen with spinning wheel.

Just suddenly we discover that our mac start giving us very hard time ( before it was little bad), but all of sudden yesterday as soon as we reboot the pc to turn on, the mac turned ON ok, but following things were happening
1) On clicking applications, they just looks like they opening but they won't
2) after every few minutes, screen goes black, and after pressing slightly power button, screen comes back but with spinning mouse

So far we able to do following things
1) Run PC in safe mode,
2) Go to disk utility and run disk first aid repair ( something like that)
3) Run Avasta AV scan ( left for whole night, but in the morning can't see where it finished)

Please assist us
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David AndersTechnician Commented:
I would remove Avast, a Google for "mac avast slow down" indicates the problem.
Mac Model? 13" or 15" 2010, 2011, etc  The hard drive cable fails on some 2010 to 2012 MBP. Symptoms can vary widely.
Google "macbook pro hard drive cable"  I have replaced 7 or 8 in the last three years.

Create a new user in System Preferences and login as the new test user (administrator) and test.  is a good diagnostic tool
Booting with "D" key will run Apple Diagnostic
Also, try resetting the SMC and PRAM.
khan1431Author Commented:
thanks guys
problem solved, this is what I did

I booted it in safe mode, then open the app store, and run the update
my OS was on Mac sierra 10.12.x and it upgraded to 10.13.xx  ( took 1 hour)
and when the system came back , all it was easy and normal.

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khan1431Author Commented:
Try to update mac os via Safe mode and it fixed the issue at the end.
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