I don't think Montresor is as unreliable as he could be. He is a murderer and allows his pride to rule him, certainly, but I don't think he is mentally unstable beyond his pride. Montresor says that Fortunato's weak point is his pride, which is ironic because it also seems to be his own, and he does provide evidence to support this. For one, Fortunato is too proud to miss an opportunity to prove Montresor wrong. For all Fortunato knows, Montresor is quite willing to find the other wine expert in town, Luchesi, but Fortunato will not hear of it. He asks, "Amontillado? A pipe? Impossible! And in the middle of the carnival!" Then, he says that "Luchesi cannot tell Amontillado from Sherry." Later, he even calls Luchesi "an ignoramus." In other words, he is certain that Montresor has overpaid for the wine, and he feels himself to be a far superior judge of wine to Luchesi as well. If he weren't so proud and so eager to prove that Montresor has been played for a fool, he would have remained safely above ground. As is, he is willing to compromise his own health in order to satisfy his pride.
Later, when the two men are within the Montresor family vaults, below ground, Fortunato
laughed and threw [an empty] bottle upwards with a gesticulation [Montresor] did not understand. . . . He repeated the movement. "You do not comprehend?" he asked. "Not I," [Montresor] replied. "Then you are not of the brotherhood." "Yes, yes," [Montresor] said, "yes, yes." [And Fortunato declared,] "You? Impossible! A mason?"
Fortunato even demands a sign that Montresor is a member of this elite organization. In other words, then, Fortunato attempts to reassert his superior social standing by referencing his membership in this sacred brotherhood and by pointing out that Montresor is not a part of the club. It seems designed not to see if Montresor really is a member but rather to make Montresor feel less important than Fortunato.
Therefore, Montresor does provide some proof of Fortunato's terrible pride (he is willing to sacrifice his health to prove Montresor wrong), as well as Fortunato's intent to insult him (by rubbing his nose in the fact that Fortunato is a mason and Montresor is not). This certainly doesn't provide a good reason to murder someone, but there is some indication that it isn't all in Montresor's imagination.