Gmapping produces false obstacles in the map at periodic intervals.

I'm using a Summit-X robot with ROS Indigo and Ubuntu 14.04. With gmapping (slam_gmapping) I map the area the robot has to navigate using a Hokuyo laser range finder. Whenever the robot moves, the created map has pseudo obstacles which are added to the map at regular intervals during the mapping process i.e. even though there is empty space, the produced map shows obstacles. I tried changing some of the gmapping parameters but reverted back to the original settings as the results were futile. When I test the robot with move_base_amcl, it indeed tries to avoid and move around the pseudo obstacles. They are visible on the global costmap as well. Here's an image with the obstacles highlighted.  It appears as though there are small unmapped regions behind the robot,which are removed with multiple iterations, but the tail of non existent obstacles remain. How do I eliminate this? Thank you.