Is Grendel an unreliable narrator? If so, why?

Grendel is a monster. No, really, he is. He's also the narrator of the novel bearing his name, which tells the Beowulf story from his perspective. The story is written in first-person prose, which allows us to get an in-depth look at Beowulf from the monster's perspective. However, can the reader truly rely on what Grendel is telling us? Is Grendel mentally stable enough to be considered a reliable narrator? Looking at the text, we see that Grendel's thoughts and observations, while moving and enlightening, are not exactly connected. Often, they are punctuated by various slips in mental stability. For example, look at page forty-five, where he says, "I clamped my palms to my ears and stretched up my lips and shrieked again: a stab at truth, a snatch at apocalyptic glee." At this point, Grendel is watching Hrothgar's hall and is hearing the thoughts and words of the men within.
Grendel's narrative is also non-linear. He often goes off on tangents, telling the reader about the past and various Danes who have previously ruled (Scyld Shielding, anyone?). He goes back and forth throughout the novel, offering bits of current happenings to drive the plot forward. 
We also have to consider Grendel's discussion/interaction with the Shaper. Is the Shaper real? Is this a figment of Grendel's imagination? Is it a hallucination? How can we count on Grendel as a narrator if we have no inkling of the stability of his mental state? We know that he is not stable—look at this quote on page 101: "I could see myself . . . running on all fours through the crowd to her . . . drooling and groveling at her small, fur booted feet. 'Mercy!' I would howl . . . I clamped my palms over my eyes and struggled not to laugh." His imagined encounter with Wealtheow gives us an indication that Grendel is not well. We see so many imagined encounters, it is difficult to know which really happened and which are figments of his mind. This is what makes Grendel an unreliable narrator.
Edition used for citations
Gardner, John. Grendel. Vintage Books: New York, 1989.