I have a Dell Latitude E5420M running Kubuntu 13.04 (now 14.04). I have it set to suspend to RAM whenever it detects a lid close. Quite often, whenever I wake it up (lid open), the computer stays active for 5-10 seconds (fully operational) and then returns to sleep without me closing the lid. I can wake it up afterward, at which point it functions normally.
I have turned the Internet inside out trying to find a solution to this, but I have found absolutely nothing.
I have noticed this behavior with Kubuntu 12.04 LTS as well (before I got a new hard drive), although it did not happen as often.
Edit: I recently updated to 14.04, but I still experience this behavior.