I have a strange problem with my site (usually only in IE) that sometimes my ad codes seem to interfere with each other. For example sometimes my adsense banner is suddenly replaced by a banner for Valueclickmedia even though the codes are completely sepearte. Or a rectangle ad will try to act like the banner ad further up the page. It's literally like they have a mind of their own, swapping back and forth. I've considered it being a caching issue, maybe a layout width issue or maybe the code isn't clean and I need to add a break, but nothing seems to really fix it. It's doing my head in.
It is possible that you are using an ID twice on the same page and they are overlapping. Either that or your condition to display them is broken. Any chance I can have a look at the site?