  To understand the origin of the dynamical transition, between high
temperature exponential relaxation and low temperature nonexponential
relaxation, that occurs well above the static transition in glassy systems, a
frustrated spin model, with and without disorder, is considered. The model has
two phase transitions, the lower being a standard spin glass transition (in
presence of disorder) or fully frustrated Ising (in absence of disorder), and
the higher being a Potts transition. Monte Carlo results clarify that in the
model with (or without) disorder the precursor phenomena are related to the
Griffiths (or Potts) transition. The Griffiths transition is a vanishing
transition which occurs above the Potts transition and is present only when
disorder is present, while the Potts transition which signals the effect due to
frustration is always present. These results suggest that precursor phenomena
in frustrated systems are due either to disorder and/or to frustration, giving
a consistent interpretation also for the limiting cases of Ising spin glass and
of Ising fully frustrated model, where also the Potts transition is vanishing.
This interpretation could play a relevant role in glassy systems beyond the
spin systems case.
