Timeline Not Accurate

I'm new to animate and everything seemed to be going well, but when I preview the movie it is much slower than it's supposed to be.  It's set at 24 fps, but seems to be playing closer to 12 fps.  Thus, the 30 second video ends up taking closer to 60 seconds.  I've read through some comments on this by people with similar problems, but the solutions do not work.  I saved and reopened the project with no improvement.  I tried changing the preview mode to fast, no luck.  I exported it to my desktop and this made no difference either as the finished product plays at the same speed.  This obviously makes it difficult to pair audio to the video as the timeline is useless.  When I did pair the audio to it, the audio plays at the correct speed, but the video continues to play somewhere around 12 fps.  Does anyone know how to correct this?  Thanks.