Are you sure all the vents are closed, sometimes it is hard to see that the bottom ones are closed all the way. Although I wouldn't think you could maintain that temp. if they were only open a slit. No it's not too much lump in the basket, it must be air getting in form some where or there would be no fire.
Was the lid shut down all the way, or was left open is the first catch and not shut down to the second catch?