The Suda has the following anecdote which seems to be taken and altered from Diogenes Laertius or something similar.
“thunderous-mouth-milling”: Eubulides says this “the eristic, asking his horn questions and discombobulating the orators with his falsely-intellectual arguments, taking with him the “thunderous-mouth-milling” of Demosthenes.
Ῥομβοστωμυλήθρα: Εὐβουλίδης φησίν: οὑριστικὸς κερατίνας ἐρωτῶν καὶ ψευδαλαζόσιν λόγοις τοὺς ῥήτορας κυλίων ἀπῆλθ’, ἔχων Δημοσθένους τὴν ῥομβοστωμυλήθραν.
ῥομβοστωμυλήθρη (lit. “thunderous-mouth-milling” (?) seems to be a misunderstanding or humorous take on ῥωποπερπερήθρη, usually translated as “braggadocio” but is more like “cheap/petty bragging”
From Diogenes Laertius, Lives of the Eminent Philosophers 2.10
“The eristic Euboulides, asking questions about horns
And discombobulating the speakers with his falsely-intellectual arguments
Has gone off, taking the petty self regard of Demosthenes with him
For it seems that Demosthenes was a student of Eubulides and was able to stop his problems with the letter ‘r’ because of it. Eubulides was also in conflict with Aristotle and undermined him a lot.
οὑριστικὸς δ᾿ Εὐβουλίδης κερατίνας ἐρωτῶν
καὶ ψευδαλαζόσιν λόγοις τοὺς ῥήτορας κυλίων
ἀπῆλθ᾿ ἔχων Δημοσθένους τὴν ῥωποπερπερήθραν.
ἐῴκει γὰρ αὐτοῦ καὶ Δημοσθένης ἀκηκοέναι καὶ ῥωβικώτερος ὢν παύσασθαι. ὁ δ᾿ Εὐβουλίδης καὶ πρὸς Ἀριστοτέλην διεφέρετο, καὶ πολλὰ αὐτὸν διαβέβληκε.