Cubase 5.1 Audio/cpu Problems, Red light and sound cuts out on repeat Options

[John and Gretche...]

We have Cubase 5.1 and are recording midi and audio tracks. We are recording guitars and voice (separately on different audio tracks) and for brief period it worked fine. BUT...now when I try to play back the CPU meter goes berserk - goes red and we lose the sound. Sometimes it plays back for about a minute and then goes. If I press play and then stop rewind and play again it goes red immediately and I lose sound. I am wondering what I might have inadvertantly changed to trigger off this reaction?

I haven't got a lot of hair and what's left is disappearing rapidly asd I tear it out! Any ideas or help gratefully received. Thanks.