it's not rare for flash to crap out especially if you do thing like 'distribute to layers' or copy library assets from one fla to another. just today i had such a problem.
but i long ago learned to save fla files with version numbers attached so a corrupted whatever_36.fla will have no impact on whatever_35.fla. at the first sign of trouble (like today's slowing), i trash the current fla and return to the previous version and work forward. problem averted almost no time lost.
do you have a working swf?