Hello Belinda Mae
The spirit that interferes with the Nechung Oracle is Nyatrul aka Nyari Tulku.
H.E. Tsem Tulku did a large article on him at
http://www.tsemrinpoche.com/tsem-tulku-rinpoche/dorje-shugden/the-spirit-nyatrul-magic-mischief-and-misery.html.He states that Nyari Tulku was "Murdered" (1899) by the 13th Dalai Lama and his Government and returned as a spirit to seek his revenge.
Nyari Tulku was falsely accused of trying to murder the 13th Dalai Lama and he believes that the Nechung gave a false trance that ultimately resulted in his murder.
It is claimed that the spirit Nyatrul took trance of the Nechung in 1933 and he gave the 13th Dalai Lama the wrong medicine on purpose.
That medicine caused his death.
So Nyatrul got revenge against the 13th Dalai Lama by murdering him.
It is also claimed in Tsem Tulku's article that it was Nyatrul that has caused the Dorje Shugden ban.
Some believe that in 1902 Nyatrul took trance of the Nechung who told the 13th Dalai Lama to fight the British army that had invaded Tibet.
The Nechung claimed that they would have a great victory.
He was wrong and Col. Younghouse who lead the British Army killed 800 Tibetan soldiers in only a few hours.
So who really is the 14th Dalai seeking his advise from?
Is it the Nechung Oracle or is it Nyari Tulku (Nyatrul) the Tulku that he ordered "Murdered"?