Starting to wonder if this is just an attempt to get some pageviews. I've checked periodically and the ads are ALWAYS there.
Just get more content on the page because it is still very plain, add much more text on your pages, spread the ads around and try not using all the same format on your ads, use different sizes within the page.
It's probably because you chose to not serve public service announcements and used white as the fill color. Google doesnt like to show the same ad over and over again if it's not performing well on your site therefore it pulls the ad after a few hours and since it has no other ad to show and you chose to not show PSAs it just loads up a solid white box. BTW loading up 3 468x60 ads in a row is.....how do you say it?.?.?.......dumb. Put a 300x250 or 2 250x250 ads in place of that mess. How do you expect people to click on the ad when they can't even read the description? Not to sound harsh but dude seriously.
I know if I load my sites using the local network IP address (192.168.*.*) it only displays 1 out of the 3 ads but when I load it normally using the domain it shows them all. I doubt that helps tho, Sorry
Same happens to me. Google has issues scanning "local" machines or domains that block all spiders/bots. I don't think it's the issue here. There are only so many ads Google has that's relevant to that site. I refreshed many times and saw the same ads over and over again. After awhile Google just pulls them and loads up a white box for a few hours.
This is NOT your problem. Sure 3 468x60's stacked is, but this is not your problem! Work on content first! Your site is bland, what do you expect the ads to be filled with if your site has 0 content?