Black Sony Vegas preview?

Basically, I imported about 23 clips of a video I'm cutting together for Youtube in Sony Vegas 10 and all was fine with editing and all. Up until I started doing the last minute tweaks, some of my videos started to go black and not appear. Now I know it's not the codec because I have the K-Lite Pack. This has happened a lot in the past and seems to only really randomly happen, but when I render out it also renders out black. Then a few minutes later, it's as though the clips have swapped roles. The ones that were black are now viewable and the ones that were viewable are now black. I have to somehow restore the view otherwise I can't render the project out. I really need help with this... Any help would be appreciated. Thanks