FCP7 Canvas turns white when attempting move text or other elements with in frame

Hi there i was just wondering if someone could help please, when i attempt to reposition text or shapes in the canvas. The canvas turns white, when i stop moving the text the canvas returns to normal. Do you think this could be a ram or processor issue? I have a mac pro desk top 2.6 quad core 4gb ram gt7300 graphics card.
Any help would be appreciated thanks!