macBook wakes from sleep when power connected and lid shut

My macbook has developed a problem where when the lid is shut and the power is connected it wakes up from sleep and then goes back to sleep and repeats this process every 30-60 seconds.

What's weird is it doesn't wake from sleep and get stuck in this loop when
—the lid is open and the power is connected and I select sleep from the Apple menu
—when the lid is shut and the power is NOT connected and is running from the battery

I recently updated my mac to Snow Leopard and put in a new HDD and since doing so the problem has been occurring.

Any ideas what might be causing them problem?