Hi Malark -
That hard line is most definitely bounds related. Go into your Particle System’s Details by clicking teh black area to the right of your emitter module stacks. In the details panel, Find Bounds and roll out the fixed Relative Bounding Box. Set a very high Min and Max Amount here. I recommend (-10000,-10000,-10000) for min and (10000,10000,10000) for max.
Let me know if this does not fix your issue -
Thank You
Eric Ketchum