CTC makerbot printing problems gap between vertical walls

hi folks new here and have just got my first 3d printerthe printer was printing fine with PLA and i have now switched over to ABS and am now having problems with printing when it prints it does not stick the all the layers of the solid walls together its almost as if it is printing 2 wall vertical wall rather than one solid one as there is a gap between them its almost as if its not putting enough ABS down or the layers are to far aparti am using makerbot desktop slicer software using 4mm nozzle printing at 250 degrees the photo below should be one solid 2.5mm thick wall all roundany advice welcomed as i am stuckmany many thanksJon

Hope someone has an answer for you. I have had the same issue on some prints.

I know nothing of your slicer but it looks like something is not set correctly or if this is an internet model it could have errors in it. Sorry can not be of more help

if makerbot works - then flashptiny will too.
use flashprint :-)
http://www.flashforge.com.hk/downloads.html