External keyboard problem

I have a strange, reoccurring problem with a MacBook Pro when attached to an external keyboard: suddenly the computer reacts to keyboard input as if the Cmd key is being pressed at the same time (although this is not the case). Just pressing the tab key, for example, brings up the app switcher, pressing the Q key closes the active application, pressing W closes the active window and all without pressing the Cmd key.

Thinking it was a mechanical problem, I ordered a replacement keyboard (Apple Keyboard with numeric keypad) but the problem persists. I think we can therefore rule out that the Cmd key is physically stuck.

The problem only happens when the external keyboard is connected. Once the problem occurs, disconnecting the external keyboard doesn't resolve the issue. The only way of resolving the problem is to log out and back in again (or restart). Pressing the ESC key has no effect.

The fact that the computer 'thinks' that the Cmd key is pressed means that any other 'normal' keyboard input it prohibited, i.e. if the screensaver is active, I can't enter my password and mouse clicks behave like CMD-clicks, i.e. clicking on a link in Safari opens it in a new window. All other keyboard input appears to be otherwise ok. For instance, I can use the arrow keys to select different menu items and the CR key to select items.

The configuration is as follows:

MacBook Pro 2.66 GHz Intel Core 2 Duo

Mac OS X 10.6.8

8GB RAM

Apple Keyboard plugged into USB port of Apple 20" Cinema display. Cinema display plugged into mini Display Port and USB port on the MBP. (It doesn't seem to make a difference which USB port is used, the problem arises on both).

I have a number of background programs running including:

TextExpander

LaunchBar

Dropbox

Evernote

BashFlash

Growl

Snapz Pro X

Thinking that TextExpander might be causing some input problems, I removed it from the system but the problem remained. I've yet to try removing any other background programs or start up items.

I use the laptop at home in the evenings in 'stand alone' mode and plug it into the Cinema display when I'm in the office. Admittedly, I plug the display and USB connectors into the laptop while it is on but this never caused a problem previously. Sometimes the problem will occur as soon as the external keyboard is plugged in, but on other occasions everything works fine for a couple of hours when the problem seems to occur of its own volition. The problem is, however occurring every day.

I hope I've thought of everything. Anyone else seen a similar problem? I'd love to get to the bottom of this.