I used to have a problem when creating banner ads in Flash. The backup gifs that Flash generates look horrible so instead of fighting with Flash I change my process.
I design banners in Illustrator. Illustrator has better tools than Flash and now Illustrator and Flash are so integrated that you can turn objects into symbols in Illustrator and they will stay as symbols in Flash.
You can also convert text into Flash text that can be edited later on in Flash if needed by selecting the text, then choosing Flash text in the menu above.
And embed the text characters which translates seamlessly into Flash by choosing Dynamic Text –> Character Embed link.
Now that you have your banner designed in Illustrator you can save it as a gif and get a crisp and clean gif backup of your banner ad then export the banner into Flash to animate.