frequent crashs using ubuntu 17.10

Question

I have recently installed ubuntu 17.10 on DELL optiplex 3020 and I increase the amount of RAM from 8GB to 16GB. But I got frequently crashs in many applications such as: chromium, firefox , visual Studio. Sometimes all the session freezed and I have to reboot the pc. I run the command memtester 16G 1 and I got this result:

Loop 1/1:


Stuck Address       : ok         
  Random Value        : ok
  Compare XOR         : ok
  Compare SUB         : ok
  Compare MUL         : ok
  Compare DIV         : ok
  Compare OR          : ok
  Compare AND         : ok
  Sequential Increment: ok
  Solid Bits          : ok         
  Block Sequential    : ok         
  Checkerboard        : ok         
  Bit Spread          : ok         
  Bit Flip            : ok         
  Walking Ones        : ok         
  Walking Zeroes      : ok         
  8-bit Writes        : ok
  16-bit Writes       : ok

the output of echo $?:

0

I'm able today to boot the sytem after the 4 fourth test and I got a black screen when I tried to reboot the pc contains many information such as kernel panic not sync. Does this problem is due to the hardware or the image of ubuntu that I installed?

Below is the syslog's file: syslog