Top Advertisement Banner Issue

Hey
I'm using openX to generate my ads on my page. Recently, I've been having a problem viewing the ads only in the top banner position. When I try to copy/paste new banner code in the ad box, it seem to work fine for about 5 or 10 minutes and then the ads stop. This might not be a vshare issue but do you think some how the codes are now conflicting or something? Like I said, its been working fine for month but the problem has just started in the last week or so. I didn't change anything. Thanks

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