Small patches of dash lines and pat...

Hi,

I have a HP vs17e monitor for about two years. About a month ago, I noticed that the colours of a photo on a web page that I was viewing started to blur out slowly. When I scroll up and down again, the photo colours becomes normal again, but the slowly to blur out slowly again.
Now I am getting lines and other pattern on my monitor diaplay.

Sometimes it is short horizontal dash lines, like "- - - - - - -" (usually flickering); these lines are a short (several dashes per line), but a few lines together (one small patches; but small patches of these on various parts of the screen). On white background, it is pink, on black background, it is green.
Sometimes, they are not lines but blur patterns that look like "@@@@" or "~~~~!" - small patches of these on parts of the screen.. When I minimise the page or document; or scroll down and scroll back again, these patches all disappear momentarily, but very soon start to appear again.

Could this be due to dust? A few months ago, I started to burn incense sticks on a shelf just above teh monitor. Could the ashes from the incense stick dropped into the top ventilation holes of the monitor? I tried to use a vacuum cleaner and put it at ventilation holes to try to **** out any incesne ashes but it didin't seem to help. I tried to open teh monitor to clean up these incense dusts if they are indeed teh culprit but I couldn't find any screws that hold the back plate of the monitor.
Or could this be due to teh graphics card? But this card was changed before - about eight months back, my screen went blank upon start up and the serviceman fixed the problem by changing tthe graphics card.

I would appreciate any help. Many Thanks.
Sam