Itunes video problem!!?

I download videos fr,om itunes and they play very badly even months after i download them. The audio cuts out and the video is all scratchy. They play fine on my Ipod but bad on my computer! I watch movies fine on Microsoft Media player and youtube but not itunes! is there a way i can make videos play smoothly and not loose audio on my computer? ( I use A Toshiba Satellite with a windows 7 operator)