Weird intermittent video card (?) problem

I hope this is OK to post in General Questions–never posted here before! I read the thread about computer related issues and it seems to be OK.

I have a Dell Inspiron 1520 laptop. It is running Windows Vista, updated with all the latest updates. The video card is a NVIDIA GeForce 8400M GS (according to the Device Manager.) This computer has had problems since I got it, but they are the dreaded “intermittent” difficulties. A few months after I bought the computer, the screen started acting up. Weird black lines would break up the screen every once in a while. It usually happened when the computer was coming out of the hibernation mode, after the screen had been shut. It started happening more and more often. Sometimes this weird white fog would come up from the bottom of the screen, looking like the inside of the computer was misting up. Eventually I had it sent off to customer service (just recently). They replaced the video card and sent it back to me. Fine and well.

Unfortunately, the techies broke my computer worse than before. As soon as I turned the computer on, I could tell something wasn’t right–the logo at the start-up screen was a weird color of pink. When Windows loaded the problem was immediately apparent. All the white colors in the screen had turned an extremely bright pink. Some other colors (possibly black?) had turned green. The whole thing was discolored. The screen also “wavered” slightly–I’m not sure how to describe it, but something was clearly wrong. The computer otherwise worked perfectly. This continued for 2 days–unlike the other issue, it wasn’t intermittent, only happening once in a while. I restarted the computer twice and the problem continued. This afternoon, I went out shopping, leaving my computer up and running. When I got back, the screen had returned to a perfectly normal state. There was no wavering, the colors were normal–it looked 100% okay. This lasted for about 10 minutes. As I type now, the discoloration and wavering is back, just like before.

I know this is likely rather futile, but does anyone have any idea what could be going wrong? I assume it is a problem with the video card, or maybe the LCD screen itself? The tech support people at Dell Japan seem to be incompetent, so perhaps I can give them some advice and actually get this machine fixed.