This is an annoying problem as I primarily use my browser for forums/message boards. When I click the 'new' link on a forum topic, it focuses the window right at the newest post. But then images start to load and push the focus around so I almost always have to scroll around looking for the newest post manually. I've only noticed this behavior on k-m browser so far.
This is normal. First, the HTML page is loaded, then the embedded images. When no dimensions (width, height) are specified for the images, the layout must be corrected to fit in the images when they are loaded. This will change the dimensions of the page (at least the height) and thus will change the position of the viewport. Gecko is correcting this position continuously as the page is loading. Finally, you do end up at the desired position when the page is fully loaded.
This is Gecko, not just K-Meleon. The behavior is the same in Firefox and SeaMonkey. I don't know what browser you used before, but in IE6, for example, things are even worse. There you may not end up at the desired position when the page is finally loaded...
If it takes you too long to load images due to small bandwidth, maybe disable image loading when visiting such a forum (just hit F9 to toggle).