I had exact same issue and every fix I was offered by Dell or online didn't help. By chance I took battery out to swap another one and that cleared it up. Don't know if that'll help you or not but simply removing the battery and reinstall it worked for me.

Call Dell for a warranty repair - they'll send a new adapter first.  If that doesn't solve the problem, the power jack or mainboard is bad and likewise will be covered under warranty.