World Editor - can't fly around

Ok ... so I am creating a new mission in TGE1.5.2, world editor. I start out being able to fly around and move everywhere, but if I add too many .dts objects it all of the sudden applies gravity in the world and i am stuck running around on the ground trying to position things ...

i searched (and searched) and couldn't find any reference to this - anyone have any ideas what is going on ?

thanks in advance,

kevin