Problem with disappearing symbols

[HowardWimshurst]

Hi guys, I have an urgent problem that I need to solve.
I have been working on an animation in flash 8. Most of my stuff is FBF but I have been using some motion tween effects for things like slow motion.
The problem is that when I motion tween movie clip symbols, thy become invisible as if nothing is there, but are present on the key frames. This is not just a problem when I export it to SWF but also in flash 8 itself. I have a preloader at the start of the animation which I thought would have prevented this.
Also, I can temporarily fix it by clicking into the symbol and scrubbing through the timeline.

Can anyone offer a reason for why this is happening or a way to prevent it from happening?
Thanks

[kmau]

My symbols tend to turn invisible when my memory is almost full, but I'm not sure if that's your problem. Try changing your movie clips into graphic symbols and see if that helps. (under properties)