Here is an article on repair
http://www.fierosails.com/headlights.htmlMine did the same but swapping units cured it. You may have a bad relay or just a run of really bad replacements.
The gear only runs up and down one side. Flipping it may help out as that side is 'new'. As Eric says, the metal gears will fix it right up.
Also '80's Firebird motors work. Use the pass motor on our dr side and vicey versey. They are at most yards an pretty cheap.