Your case is a controversial issue,
1> it is not a good practice to have the .fla source file that big even if you have a reliable PC with 5 gb RAM, WOW 300 MB!
2> Flash carries on occupying more and more space in your RAM's while you work with the file, because it registers your work steps so that you may return back in steps which needs more and more RAM space.
3> Try to eleminate unnecessary files in your library.
4> try to splet your work into sub movies and use the loadMovie() methods and if you have video files try streaming and externally loading them instead of placing all of your work into single FLA file.
5> resize your images before importing them into flash library.
are you doing a work for TV or Website?
I am working for a website, there is no video just animations. I am not too trained on flash, as in i can only make things work if i draw out every other frame. there is no video what so ever in the site and i go though i whipe out my memory evey so often just because. though i dont see how it would cause flash to crash as soon as a touch something to make a change.
In settings you could try turning down the undo-redo memory.
As you are working for a website, then you don't need a 300 Mb fla source file.. When I was new to flash I was doing such mistakes like importing images and videos as it is without preparing them to be imported to flash. Even a 20 MB FLA file is still large.
well, my advice if you still have your projects assets (images, audios...etc) then delete your current bad FLA and start a new project, and then do the following:
1> resize and/or rescale your photos (you can use Adobe Fireworks which has a batch processing or you can download Microsoft powertoy resizer which resize only JPEG photos).
2> try to encode your audio files if you have one using an editor like WAVPAD that can edit and compress audio files and convert them to any format you wish. I recommend 64 or 128 mp3.
wish you success.