In 'Petronius and the New Non-Literary Latin', J.N. Adams refers (p. 12, n. 7) to a single occurrence of the frequentative adiutare (contrast adiuuare) 'in a letter'. He does not give a precise citation, but refers to p. 80 of his 1977 work on Claudius Terentianus.
The instance in Cicero is ep.fr.11(10)3, according to OLD, which cites P.Mich.471.28 (Claudius Terentianus: images), Lucr.1.812 and Sallust's epistle de re publica 2.12.4, as well as Plautus and Terence, as mentioned by Adams.
This fragment can be found in the OCT M. Tulli Ciceronis: Epistulae, III (1902 ed. L.C. Purser [not paginated] and 1958 ed. W.S. Watt, p. 171). Arusianius Messius vii, p. 453.23 noted (on adiuta hoc illos) Cicero Ad Axium, ex libro I: si tu aliquid nos adiutare potes.