Computer Monitor Discoloration?

I recently purchased a pc from a friend. It worked just fine at his house. It has been about 2 weeks now and i have started to notice some weird discolorations in the screen. Mainly it's the color black looking like a fuzzy red. Like the bars on a youtube video or the black of a furnace in Minecraft. Also sometimes there are areas that are fuzzy or have weird discolored squiggles in them. The weird thing is that this problem doesn't happen 100 percent of the time and that's what makes it weird. Sometimes the wide screen bars or the black furnace or the orange button look exactly as they should but other times this discoloration happens. I'm running windows 7 on this pc and it's from 2009. It works perfectly other than these random occurrences.