Unwanted lines on semi transparent object drawn with pen tool

Hi everyone,

I am getting unwanted lines on the edge of a shape that I have drawn using the pen tool.

The lines only appear when I bring down the opacity of the shape (to make it semi transparent) and seem to follow the adjustment handles of the anchor points – I believe this must be somehow creating the problem.

I've changed the opacity and colour various times but the problem persists. Please see photos. 

Please could someone advise me on how to get rid of these lines.