I've got an Aluminium iMac (320Gb HD) which I've now had for about 2 weeks. A couple of days ago, I set up Time Machine, backing up to an external 500Gb drive via Firewire 800. Now about every 5 seconds or so, the machine 'stops'. If I'm typing at any speed, the odd letter is missing as the machine pauses and doesn't process it. Also, moving the mouse causes momentary freezing which is SO annoying. However, I've just turned it off and it's still doing it. It wasn't before TM was set up. Any ideas?