Looks like SpeedStep in turned on and this is normal.  The cpu will throttle up when needed and throttle down when its not needed to save on power and heating.

I would look in the bios and disable it there. Also look for any setting for C1E, that will also cause the CPU to throttle up and down.

This is a design function in the chip to help conserve some power when the CPU is in a idle conditon 

Under power options in control panel, it's set to Home/Office Desk, which turns Speedstep off.

http://en.wikipedia.org/wiki/SpeedStep#Operating_system_support

Here's a screenshot from CPU-Z

http://farm4.static.flickr.com/3228/2617015614_c5d3143871_o.jpg

I'm running Windows Xp Home

I couldn't fine C1e settings in the bios. I know I have speedstep turned off in the control panel. I'm really starting to think that one core semi-died on me :( Luckily if that's the case it's till under warrenty.

What Os are your running?

Pick up a copy of CPU-z from Here

Here is some info from your manual

 

http://support.dell.com/support/edocs/systems/inspd530/EN/OM/setting.htm#wp1100889

In the screenshot of CPU-Z you posted it is clearly throttling down to a 6x multiplier thus resulting in the lower speed. Changing the settings in the power options will not effect this.

When the CPU needs extra under load the multi will increase as needed and get your rated speed.

 

Thus far your system is running as it should.  If the cpu needs for power it will throttle up and if not it throttles down and nothing to worry about.