Running Too Fast!!??

Recently, I've downloaded some media files (mpeg) that Windows Media Plyer starts and the progress indicator moves along very fast, and then the playback gets choppy. When I grab the progress indicsator and move it backwards and hold it the file plays normally until that point, but as soon as I let it go it races to the end, and the movie gets choppy again. Has anyone seen this before, and does anyone have an idea on how to correct. The files in question are advanced training in vehicle extrication techniques for firemen. I really wanted to use them as training aids.