Most likely it's because your shape tween is too complicated and tweening it from one key to another produces way too many vectors for your processor and graphics card to handle. Sometimes rendering that into a movie clip will get rid of that and it will play fine (if it doesn't crash while rendering). If your end result is a swf though, you're probably going to have to simplify the logo or what you're trying to accomplish with the tween to keep the frame rate up.
Can you post the file?
thank you, i think thats the problem because it crashed when i made it a swf file and tried to view it, i have to redue the design and im already a day late handing it in *ugh, what i get for procrastinating on school work.....but anyway, thanks for saving me anymore time trying to figure out whats wrong
Happy to help!