The thing is: I had this problem before and solved it by using media player classic instead of windows media player - it did the trick.
Now it doesn't anymore (after the reinstall). I tried the Overlay Mixer in mpc's settings - still no video on my TV screen and mpc and wmp both play the video very slowly.
It is not a codec issue, the video is dislayed perfectly on my PC screen.
I have downloaded an older version of mpc to no effect.
I seem to have to restart now in order to play the video flawlessly when I change the output.
I have updated video card drivers.