I have had the same problem after I exported a project containing XDCAM footage and 7D footage converted to XDCAM. I could not figure a way to slow the footage down considering my sequence settings were all at proper speeds and rendered at a full frame rate. I tried clicking the “enable” on all tracks. A window pops up saying you will lose your render files. Click ok. Then play the clip. Then enable all the video tracks again and try playing them. This saved my project and the upcoming due date. Let me know if this works?