Interestingly enough, I have just tested it on my work computer and experienced the problem.
I do believe your problem originates in the fact that you're using tables and absolute positioning. Let me explain a bit.
Tables are by default rendered sequentially meaning that table gets rendered then it expands to fit all the content. That is why you see a lot of "shaking" when you load pages that rely on tables a lot -- original tables expands to fit all the pictures that are loaded after the rest of the text.
Same thing is happening on your page, with one exception. Your first table has no content before the picture is loaded, so it is 0 pixels high. Then you have two paragraphs following that. And then your absolutely positioned contentLeft. Now, before the header picture is loaded, the contentLeft is positioned absolutely at the position under zero sized table and two paragraphs. Then picture is loaded and while in normal circumstances would push the content lower, this does not happen in your case, since it was positioned absolutely, or taken out of the flow.
Solutions which are more benefitial to you than not are plenty.
I would start with removing the table from the header. It is just a picture. You do not need to put one picture in a one cell table. At most, you could use a div or even just the image.
Then again, the other problem, being absolute positioning is also best removed. I checked your page without that declaration and it looks the same. It is completely superfluous -- and rather than helping is just confusing browsers.
The rest of your code also needs work. The second table in the header also seems to not be necessary but if it is, it should definitely not be in a paragraph.
Try the changes I suggested and tell us how it went.