PC screen freezes randomly!!

My friend's PC is giving a random problem. The display screen freezes with scattered lines. This happens randomly, sometimes it happens even when the Windows boot process is going on and can happen even after 15-20 minutes after the boot while working on PC. Some suggested it could be dust on RAM, so we have wiped the RAM slot and the RAM of any dust, even cleaned the dust off the mobo, but problem persists. The PC has AMD A4 5300, biostar hi-fi A55S3, kingston 1600 MHz DDR3 4 GB. The PC is not used for any high-end gaming and doesnt has a gfx card.
What could be the problem here? How to rectify that?

If sometimes PC works properly and sometimes not then it seems to be issue of moisture. Get hair dryer and be liberal to dry your PC and see if it helps. Target every component, motherboard, RAM or any other card used in your PC.

Thanks Savera for raising that point. We tried that but problem persists. I suspect it could be a RAM issue or a mobo problem. :(

It could also be a RAM related issue. Pull out the RAM stick and change its slot. If you have multiple RAM sticks, then start the computer having just one RAM stick to figure out which RAM stick is giving problem. If you can then try attach a different RAM and see if the issue is there or not.
As you say its a random problem, then it could happen due to overheating as well. Contact Biostar support/customer care for their help.