Hi I'm new here. I have problem that my computer especially if i playing games , it shows many weird dots, and line. I've try to change my monitor from an old crt to lcd but the weird dots and line still there . can anybody help?

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It looks like a problem with your graphics card. Either it is failing or overheating.

Do you have an discrete (add-in) graphics card, or are the graphics on the motherboard?
If it's a seperate card then check for dust build up and it it has a fan make sure it's clear and running ensuring that the card is not overheating as Mailpup has suggested.

If the card is fine or the graphics are onboard the affected component will need to be replaced in the not to distant future-if it's under warranty then all good. If it isn't then you can try downclocking (lowering the Core and memory frequency of the graphics), it may alleviate the problem so you can use it without it artifacting (the lines and squares) until you can replace it.
There is no practical way to fix the problem.

This site has a gallery of showing typical artifacting http://www.playtool.com/pages/artifacts/artifacts.html
The examples are more severe than those you are experiencing but artifacting always gets worse

The hotter your GPU gets, the more trouble it has creating 3D images. Under load, when you start seeing artifacts (the dots and lines you've noticed) it's most likely because your card is running too hot. This can also cause random restarts and freezes, like when your PSU, RAM, or CPU is running too hot.

Download Speedfan , a small utility to measure system temperature (among other things). Your processor temperature should not exceed 55 degrees Celsius. Your GPU temperature should not exceed 75 degrees Celsius. Make sure all your case fans are running at 100% speed.

You didn't provide any information about your system. But, if you have dedicated graphics (a video card on your motherboard, rather than a chip built into your motherboard that processes your graphics), then download Rivatuner, a utility you can use to unlock higher fan speeds on your video card. Set your video card fan for 100% speed.

If your GPU is still running too hot, pop the case open and blow a fan directly onto the video card when you are going to put it under stress. A more permanent solution is to buy a larger case, buy a heatsink your video card.