So you are linking to a DHTML drop-down? Or have you created drop-down text using some other method?
Generally speaking, sometimes we do find that RoboHelp tries to be helpful like a three year old when you have made edits to the HTML and switch back to Design view.
We need more information though to help. For example, exactly how you created the drop-down text and whatnot.
I used the link_to_dropdown.js script by Wvan Weelden that Peter Grainge linked to on his site. I think I may have it working now, though I haven't tried to publish yet. At least I can switch from HTML to Design without having it stripped out at the moment. I'll keep you posted.