Acer Aspire 5935G Problem

I recently bought an Acer Aspire 5935G (bout 4months back).

It has dual videocards, one intel onboard and another ATI 4570 for entry level gaming ( I play WoW on EU Servers and found it to be not a bad video card for WoW). You can switch from a video card to another true a button (Power smart)

Basically, I turned on the ATI video card to do some gaming and to my surprise the screen was showing some vertical lines ( really thin lines showing like artifacts flashing) I took the laptop back to the shop and a few days later I got a call saying it was the video card and another video card was ordered (under warranty of course). I took the laptop back home till the video card arrives, then they will give me a call to bring the laptop back in so it can be replaced.

So I switched on the laptop and used it for a while on the onboard videocard which at the time was not giving problems, and after a couple of houres, the problem started being displayed on the onboard video card, now i understand a bit on computers as i am an entuisaist myself. Before I call them up next monday and speaking with the technician who worked on it ( who fortunatley I know as i used to work there myself (at the shop )). I am wondering what you might be thinking the problem is, my guess is the mainboard as it is displaying the same problem on both video cards, I checked the cables between the monitor and the mainboard and both are good, and the technician checked them himself.

I was thinking of requesting another laptop as if it is a mainboard problem, which it most probably is, I as a customer would not like a brand new laptop with the mainboard changed just after 4 months.

What is your opinion? Thanks