Sometimes he ________, words coming unconsciously from him in grotesque exclamations.

To 'prattle' means to talk at length in a foolish or inconsequential way, which perfectly describes 'words coming unconsciously in grotesque exclamations.' The word suggests mindless, excessive speech without substance. 'Condoled' means expressed sympathy, 'stagnated' means stopped progressing, and 'oscillated' means swung back and forth - none fit the context of unconscious verbal outbursts.