I'm pretty new to printing and while I have worked out a lot of bugs I cant seem to fix this one. On most prints it starts out fine and after several passes the lines become jagged and incomplete. I have tried changing filament,temp,nozzle height,speed all with no luck. The pictures show the issue. Thanks!

http://tinypic.com/r/33a74mt/9
http://tinypic.com/r/20hawi0/9
http://tinypic.com/r/257kepw/9