funny to answer my own question. But the screaming and yelling at the screen finally helped:attn:
So for anyone who has the same problem here is a sollution. The trick is to work without an expressin. So you delete the default expression maya attaches to the frame extension.
Next you keyframe the frame extension, put the first keyframe where you want your clip to start and the last one according to the number of frames your clip has.
Then open the graph editor select the frame extension curve and set it to oscilate or infinity.

have fun

Originally Posted by farbtopf