Need Help With Jittering Objects While Building

hey all, just need a bit of help here, i am trying to build on a bit of land i have, and sometimes when i am positioning single objects *just single prims* they start jittering all over the place. i will put it in to place and when i stop clicking on it it moves around a lil bit and goes out of place, i have tried adjusting it using the red green blue coordinates, and it still does the same, me and a few other people have had this problem and am not really sure what to do. any ideas anybody?..... if anybody knows i would owe you big time!!

thankss

Sounds like they're physical.

physical? dont understand what you mean