If you are not using a default model I think your problem might be related to a mistake I also used to make when I started with 3Dgamestudio...

It seems like the bounding box is alot bigger than your visible model, to fix this follow two steps:

1. Open you model in med. You probably have the "feet" of you model at the center of the grid, correct this by placing the model CENTERED at the origin of x,y,z in med (this means that that your model in med has to have his stomach at the center of the grid). If you dont understand what I mean or you want it to be exactly at the center there is a way to achieve this automatically in med. Load your model in med, open the "edit" menu -> "transform model global" -> and click "center model".

2. In you model's action function add this line at the beginning to make sure the bounding box used for detecting collisions is exactly the same size as your model:
c_setminmax(me);