Horizontal lines across the monitor - BenQ

[nooke]

Hello,
I have BenQ G2420HDB monitor (using DVI-D cable), which is 4 years old and it was great till now. "GT 630" graphic card and i think its not the problem with it.
Few days ago, few minutes after i turned it on some weird flickering horizontal lines were showing up on screen,
it was like that one minute or so, not longer. When i turn off and turn on monitor during this problem it shows those lines when its benq logo on screen, so i think its not about graphic card.
After that everything was normal and about 5 hours later happened again, same way and it happened couple times more randomly and it didnt last longer than one or two minutes
What could be the cause of this problem and possible solution/s? Thank you

 

[nooke]

Thank you for your reply, few minutes ago it got really bad.. shaky and there were more lines then before and it lasted around 3-4 mins
i will try your suggestions, thank you

 

[nooke]

This morning when i turned on monitor i heard some flickering sound at the back of monitor.. Few minutes later it stopped and i went to the kitchen and 15mins later when i came back i saw black screen, but the monitor was on..
I turned him off and then on, picture came monitor logo and from pc and after 2secs disappeared, black screen came on..
Tried again and got same results