open task manager details to look at performance of cpu and ram.  What is cpu frequency and how much % memory is being used.
If memory is only 4G, upgrade to 8/16 Gb.
Try a fresh clean install of Windows 10 to r/o malware/bloatware issue. 

I bought this unit from a friend neraly one year now which I dont know before if there is an issue like this, I am upset to him.

thanks for the reply..my processor is intel core i5 6 gb ram, I check in task manager and it shows the used memory % is in normal. 

Another potential cause of random freeze is a dying hdd with more n more errors

I had an optiplex like this just does not respond crisply even w 8 gb ram and doing simple task like opening Edge.  I discovered eventually Windows can not properly create new partitions in the hdd for a clean install. You can try a clean install using another spare hdd or ssd to see if it improves dramatically.

ok if have extra I will try to change hdd maybe..I also doubt of the hdd because I just change a while a video card I bought from online but still the same situation..