its only started happening the past month or so, and is very random. the past 4 or 5 years there's not been any problems with it, and the sample rate of 48k. As I say an app restart solves it thank goodness lol. Not that it should matter, but the desk is also at 48k sample rate. I haven't tried unplugging the headphones jack and replugging it to see if that would cure it, but will try it the next time it does it.
Thanks for the reply Peter.