problem playing audio

I have Cubase 6 Artist and am having problem playing audio files imported into projects. When i play the audio file it starts playing for a second then cuts out. The red bar for the disk cache overload then shows on the transport panel. Sometimes after this happens the audio files will just start playing again with no problem but other times i just can’t get them to play. Any ideas what the problem is? I never used to have this problem. Only noticed it recently.

And yet i have had no problems today playing the audio files in the same project. Very strange.