Display flickering & random black screen

Hi All,

I have installed Lubuntu 20.04 on my old desktop. I am using BenQ monitor which support 1920x1080 mode. But, motherboard GA-H55M-S2, does not have HDMI support, so I am using VGA cable.

Right now display is flickering (small white lines appearing and temporarily screen goes black for couple of seconds.I am not sure if this hardware or software issue. So need some help.

System:    Kernel: 5.4.0-40-generic x86_64 bits: 64 compiler: gcc v: 9.3.0 Desktop: LXQt 0.14.1
                 info: lxqt-panel wm: Openbox 3.6.1 dm: SDDM Distro: Ubuntu 20.04 LTS (Focal Fossa)
Machine:   Type: Desktop Mobo: Gigabyte model: H55M-S2 v: x.x serial: <filter> BIOS: Award v: F4 
                 date: 08/20/2010
CPU:      Topology: Dual Core model: Intel Core i3 540 bits: 64 type: MT MCP arch: Nehalem
               rev: 5 L2 cache: 4096 KiB 
               flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 24533 
               Speed: 1467 MHz min/max: 1197/3059 MHz 
               Core speeds (MHz): 1: 1467 2: 1467 3: 1467 4: 1467 

Graphics:  Device-1: Intel Core Processor Integrated Graphics vendor: Gigabyte driver: i915
                  v: kernel bus ID: 00:02.0 
                 chip ID: 8086:0042 
                 Display: x11 server: X.Org 1.20.8 
                 driver: modesetting unloaded: fbdev,vesa 
                 resolution: 1920x1080_60.00~60Hz 
OpenGL: renderer: Mesa DRI Intel HD Graphics (ILK) v: 2.1 Mesa 20.0.8 
               direct render: Yes

Audio:     Device-1: Intel 5 Series/3400 Series High Definition Audio 
               vendor: Gigabyte 
               driver: snd_hda_intel v: kernel 
               bus ID: 00:1b.0 chip ID: 8086:3b56 
               Sound Server: ALSA v: k5.4.0-40-generic 

Network:   Message: No Device data found. 
                 Device-1: NetGear WNA1100 Wireless-N 150 [Atheros AR9271] 
                 type: USB driver: ath9k_htc bus ID: 2-3:2 
                 chip ID: 0846:9030 serial: <filter> 
                 IF: wlxe0469aaa927e state: up mac: <filter>

Drives:    Local Storage: total: 480.67 GiB 
               used: 15.04 GiB (3.1%) 
               ID-1: /dev/sda vendor: Seagate model: ST3500418AS size: 465.76 GiB 
               speed: 3.0 Gb/s rotation: 7200 rpm 
               serial: <filter> rev: CC46 scheme: MBR
                ID-2: /dev/sdb type:
                USB vendor: SanDisk model: Cruzer Blade size: 14.91 GiB
                serial: <filter> rev: 1.26 scheme: MBR 

                Partition: ID-1: / size: 457.45 GiB used: 15.00 GiB (3.3%) fs: ext4
               dev: /dev/sda1

Sensors:   System Temperatures: cpu: 46.0 C mobo: N/A 
                 Fan Speeds (RPM): N/A 

Repos:     Active apt repos in: /etc/apt/sources.list 
           1: deb http://archive.ubuntu.com/ubuntu focal main restricted

Info:      Processes: 173 Uptime: 48m Memory: 3.59 GiB used: 2.09 GiB (58.1%) 
             Init: systemd v: 245 
             runlevel: 5 
             Compilers: gcc: 9.3.0 alt: 9 clang: 10.0.0-4ubuntu1 Shell: bash v: 5.0.16 running 
             in: qterminal inxi: 3.0.38

The obvious thing to check (at least to me), is to check the contacts (cable & where it plugs in both ends) are clean & not loose, and cable is secured firmly.

Oops. cable was little bit loose, I tightened the screw, and flicker is less. There is screen tearing But on QTerminal I can see horizontal white lines going through top to bottom like in old tv sets. And little small white lines after fonts. May be I should get some high quality VGA cable and try again.

So, I took VGA cable to nearby repair shop and got it tested on monitor. There was no tearing or flickering or white line issue. Looks like issue is not with the cable. I still see white line and screen goes blank temporarily for not that frequently any more. I should start looking into hardware issue now.

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