When does it jump? Is it on hovering over links. or at page load? Are you using clear:all somewhere on the page? Are you using % widths or ems? If so try leaving some free space between them. Mail me if you want more help with it, I don't visit here all that regularly :)

thanks hostyle... I'll probably be in touch alright!

ps - it jumps on page load. And mostly pixel-perfect type layouts I think. I'll get back to you on the clear:all!

Thank you so much!

I've been having this problem for months with various websites and using inherit for the right hand column along with a large margin to clear the left column has resolved the random jumping around.

Your my saviour!
Thanks!

Previous:

#menu {
float: left;
width: 200px;
}
#content {
float: left;
}

Now:

#menu {
float: left;
width: 200px;

}
#content {
padding-left: 230px;
float: inherit;
}