Hi Frits,
That method does a soft redirect which means code after the redirect may still be executed, you should always follow a call to that method with return; to prevent any other code below it from executing, or else use Response.Redirect(url,true); which does a hard redirect.
Also only use redirects when you really need them, ie don't use buttons that only do navigation by redirecting, it ok to redirect after completing some processing in a button click event but if all the button does is redirect it is better to replace it with an ordinary link and avoid the redirect.
If the problem only happens in IE then you may be able to solve it by using page transitions in the <head> of layopiut.master like this:
<!--[if IE]>
<meta http-equiv="Page-Enter" content="blendTrans(Duration=0)" />
<meta http-equiv="Page-Exit" content="blendTrans(Duration=0)" />
<![endif]-->
Hope that helps,
Joe