emusonacid! legacy

I'm loading in samples into my E6400 Ultra from it's internal hard drive and after 5 minutes it starts clicking. When I recently did a gig firing a vocal sample over 4 bars, when it clicked the sample stopped and continued after the click.
Any ideas?
I have 4 Ultras and use them regularly on the road. They are never any trouble and are the best you can use for reliability.