This could be a long shot as well, but i had that problem as well, when my footage I was trying to play was on a faulty SATA hard drive. Try unpluging any SATA drives you may have then reboot the computer, play the footage from another drive, it worked for me. As long as my SATA drive is not plugged in, Vegas works perfectly, I also got a lot of “delayed write file” errors with that drive.