There is a bug within all offical versions of Scratch on all platforms that maninfests itself badly on low power devices such as the RPi.
MathsWhizz from the Scratch forums has developed a solution which only just involves a few small changes to the Scratch.image. A modified image for use on Raspbian based systems is available to download here
Your Scratch.image file is at /usr/share/scratch.
I suggest a sudo cp /usr/share/scratch/Scratch.image /usr/share/scratch/Scratch.orig.image to give you selves a backup first and then sudo cp /home/pi/ScratchM.image /usr/share/scratch/Scratch.image
The modifications to nothing else than stop the variable values being updated on the screen unncessarily . No changes are made to any of the core code of Sratch and any Scratch scripts created wiith this version of Scratch are identical to any scripts produced by the default Scratch image.