My name is Todd and I'm a customer advocate at Dell's corporate headquarters. My team was created to help customers via blogs and forums.
If you haven't checked yet you may want to check the reed switch on that system. There is a small switch that when depressed (i.e, closing the lid) it puts the system into sleep mode. If that switch is stuck or sticking it could cause the problems you're describing.
You can check our support site, http://support.Dell.com
, to review the manuals for your specific system to see how to check that switch.
Let me know if this helps.