old movies/series not streaming with avisynth/mencoder

[shelfi]

I've been using ums for a long time but I always encounter a problem. If I stream a new movie using avisynth/mencoder with interpolation it works like a charm. But when I want to watch an old movie it doesn't work. My TV says File not recognized. It is driving me mad because I like the interpolation option but it only seems to work on new movies.

Anybody has a solution for this problem?