suddenly VERY SLOW

Hi everybody, these days it seems to happen more often: I'm writing a piece in Musescore and suddenly, for no apparent reason, the programme gets soooo slow, that I can hardly work any more; impossible to draw F's or p's to where I want them to be, everything si so slow! I cannot figure out why this is. Most of the time, the problem solves itself: suddenly it's ok again (for no reason as well), but sometimes I have to start anew with the same piece of music because it won't work anymore. Would anyone knowwhere the problem comes from and how I could solve it???