David,  this might be worth a look
http://support.apple.com/kb/DL947

If still no joy try opening the Activity Monitor in the Utilities Folder.
Set it to show all processes and sort the data by %CPU by clicking on that 
column.
When the stall happens look at the monitor and see what is using the CPU.

On 29/03/2010, at 2:42 PM, David Noel wrote:

> 
> -- Hi All, can anybody throw any light on the following?
> 
> -- We lost power during the Great Hailstorm of March 22, and were off
> for 4 days. When power was restored, I was pleased that my Intel iMac
> (on 10.6.2) was able to power up again apparently normally. But since
> then, with all applications (eg writing this email), for no obvious
> reason the computer has at times gone into a 'pause mode'.
> 
> -- With this, during actions such as scrolling along a movie or
> switching to another application, the screen more or less freezes and
> the coloured wheel appears. After about 24 seconds the wheel
> disappears and normal service is resumed. It's bearable but annoying,
> without apparent ill effects in the long run, sometimes happening
> every minute or so, sometimes going half an hour or more before
> showing up.
> 
> -- I have the widget which shows CPU, disk etc usage and can't see
> anything special with it (though invoking widgets during a 'pause'
> usually shows everything without data, and sometimes the CPU widget
> doesn't appear at all).
> 
> -- Any thoughts, please?
> 
> Cheers --
> 
> David Noel
> 2010 Mar 29
> 
> 
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