Video in Power Point 2007 Issue

I am working with a Power Point producer and have provided 9 video clips for a Power Point 2007 presentation. The videos have been encoded as .wmv files. Here are the playback issues we're having. The first 7 videos play back-to-back--but are separate videos to allow the presentor time to talk about them. They are short--20 seconds to max 5 minutes. When the slide containing the video comes on, there is a still frame of the image. When you advance the Power Point to play the video, it is noticeably darker. I could live with this. The big issue is when you advance to the next video and play it, there is a brief still frame of the previous video before the current video plays. I'm wondering if you need to have blank slides in between? Has anyone expereinced this issue? Thanks in advance for any tips. The project is due Wednesday!