On pg. 444 I address the variation in the vowel before -nd- in gerunds and gerundives formed to 3rd and 4th conjugation verbs. In addition to the observations recorded there I should also have mentioned the discussion of Alfonso Traina in the 2nd edition of his book, Forma e suono. Da Plauto a Pascoli. Bologna: Patron, 129–143. Based chiefly on an examination of the invariant cases of transmission in the manuscripts of Terence, Traina establishes the following tendencies:
After -u, -ṷ-, -qu- the form is always -end- (vivendus).
After a syllable containing o or u the form is predominantly -end-
After i the form is predominantly -und- (faciundus).
Thus it seems likely that the choice of vowel was governed largely by avoidance of two segments with identical values for roundness.