Landscape problem

[qbawoo]

Hey folks,

 

I'm trying to clutter the simple landscape I just created with a couple of buildings... however, the render window goes ape on me when I try to move these objects around; they tend to disappear, or the landscape itself starts deforming. I'm a bit frustrated with it, been fighting through the whole world space/ heightmap edition for a while now, reading all the tutorials I could find... none mentions the issue I'm experiencing, though.

 

Any idea what I'm doing wrong? Cheers in advance