I had similar problems with IE and google's Adsense. Turns out it was something to do with the css. Never tracked it down exactly, but resolved it with a different layout Perhaps that doesn't help too much, but what kinds of consistent symptoms are you seeing? Is it the same on the same pages all the time or random? Does it hapen in both IE and Firefox, etc.?
It happens in both IE and Firefox and it happes on all pages. Do you think if I emailed YPN they would help?
That's what they're supposed to be there for. As long as you're not asking them for any hints, which it sounds like you wouldn't be, then they'll probably help.
sometimes it is not the problem with IE or Firefox this may be due to shortage of ads in that category. I sometimes see 3-4 ads from the same advertiser....the only thing you can do is that change it to someother similar category
If you do not use ad targeting, I would suggest adding that. It might fill in the extra boxes that don't show ads right now.
I actually am having the same problem on a couple of my websites. It mainly deals with the MAIN PAGE of my website. There is a space for a 468x60 ad that is 95% BLANK on the main page, but if you move to any other page it displays an ad. Very weird.