I was surprised to find not just one, but two fifth-foot spondees in Ausonius' Versus Paschales, found under heading IV in Green's edition--a poem of only 31 lines. Moreover, the two lines come in quick succession, only separated by one line. Here are lines 21-5:
trina fides auctore uno, spes certa salutis
hunc numerum iunctis virtutibus amplectenti.
tale et terrenis specimen spectatur in oris
Augustus genitor, geminum sator Augustorum...
Update: Upon looking through Green's commentary, I realized I missed a third spondeiazon, which comes only three lines after the last one mentioned:
...omnia solus habens atque omnia dilargitus. (28)
This really is remarkable--three spondeizontes within six lines?
For readers interested in the interpretation of this poem (and it is, to my mind, a very interesting poem), Green points us to the treatment of J.-L. Charlet entitled 'Theologie, politique et rhetorique: la celebration poetique de Paques a la cour de Valentinien et d'Honorius, d'apres Ausone (Versus Paschales) et Claudien (de Salvatore)', found on pp. 259-87 in La Poesia tardo-antica: tra retorica, teologia e politica (Messina 1984).