SWF goes to "Not Responding" for brief time


In the intro sequence to my file, I have titles fading in and out using a black square over the top of them which tweens from Alpha 0% to 100%. The first title works just fine.
But when i comes to the second title, sometimes just as the transition is about to begin, the SWF file crashes for about 15 seconds before continuing. I can’t see why this is happening, there is no actionscript on the frame that this occurs. I’ve also tried publishing as an EXE file but the same thing happens. Also it’s worth noting that this happens only most of the time, not every time.

Can anyone tell me what’s going on?