I've had a look at you page when the errant ad is showing in Google Chrome.
By using Google Chrome's "Inspect Element" (from right click menu) it looks like you could cure the problem by increasing the width reserved for the ad from 300 to 340.
That is change:
style="padding: 5px; text-align: left; float: left; width: 300px;"><!-- PUT THIS TAG IN DESIRED LOCATION OF SLOT MainArticleAd300x250
style="padding: 5px; text-align: left; float: left; width: 340px;"><!-- PUT THIS TAG IN DESIRED LOCATION OF SLOT MainArticleAd300x250
For some reason that I've not been able to determine Google Chrome's "Inspect Element" is showing for the computed style:
Where this spacing has come from I don't know. If you could eliminate it, that might be a better solution than increasing the reserved width.
The reason this ad is casing a problem where others don't is that it's image uses the full space available of 300 by 250.