Why is my computer monitor flashing colors?

My computer screen looks just fine when I boot up and get on the internet. Within a few minutes it starts to flash weird colors, then settles on blues and yellow-green. After looking at the blue-yellow-screen for a while, I look away and everything in the room looks redish! My cones and rods are freeking out! Is it my monitor? Video card? I've done all kinds of system tests and everything is "OK". What could be wrong and is there a cheap fix?