Give your li.drop_down fixed width. That should clear it. I donβt remember the exact reason for this, but I just tested it on your site and it works.
The calculated width is 217px right now, so try this.
.drop_down { width: 217px; }
EDIT: It looks like you have .drop_down2 and .drop_down3 (maybe more). You will need to do the same for those. I would suggest assigning a common class for each.
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