iBook - Blinking/screen turning white on iBook

I have an iBook G4, 1.42 ghz processor, 1.5GB ram that's almost 4 years old. I bought it refurb from the Apple store just over 2 years ago.

A few weeks ago the screen started blinking and turning white for anywhere from a few seconds to a minute. It's completely random. At first I thought it just did it when I had a lot of programs open, but now it's done it even with no programs open.

I took it to the Apple store and the guy there said it was either a logic board issue or a ram issue. So I bought new ram (upgraded from 512mb to 1.5gb). It made the computer much faster, but it still blinks.

Then I ran the hardware test on the install disc 1 twice. No problems were detected either time, even with the logic board.

What else could it be?