System pause/stutter every second, even in BIOS menu

Hi,

My Dell Inspiron 1720 (2GHz Core2 Duo, with 2Gb RAM, GeForce 8600M GT) has an irritating system pause EVERY second.

I have read about lots of 'causes' for this including the WLAN card, nVidia drivers, etc

In Windows it shows itself as a brief (maybe a 10th of a second) pause in the mouse movement, and if playing audio it sounds like feedback or a "BRRRRRB".

The weird thing is, it happens in Safe Mode, and after booting from a Windows System Recovery DVD, and after booting from Windows 7 install DVD.

I thought it might be a failing HDD (Dell Diagnostics reported HDD failiure imminent) so I removed the drive and booted from the DVD into Windows 7 install - same problem - a mouse movement pause every second.

So next, I removed EVERYTHING I could - HDD, extra RAM, WLAN card, battery, and then booted into BIOS.

It's a lot harder to notice because of the text-based display but when scrolling up and down the options list on the left I can still detect a pause in the cursor movement.

Booting up into Windows 7 Install from the DVD, with EVERYTHING removed as above, it still has the same problem.

Does anyone have any idea what could be causing this?