A general approach to construct a model potential with parameters fitted to nb initio energy surfaces, including many-body nonadditive effects, developed in our previous works is applied to the Ag-6 cluster. A molecular dynamics study of structural and dynamical properties of this cluster is performed using such a potential. Two new stable two-dimensional isomers with C-2 upsilon and C-2h symmetries are identified as local minima of the potential surface using the simulated quenching technique. An analysis of the thermal stability as a function of the cluster temperature reveals interesting features in the melting like transition of Ag-6. A two-step isomerization phenomenon is observed : at temperatures around 300 K, the cluster structures fluctuate among two-dimensional isomers, at higher temperatures (500 K), fast transitions occur between two- and three-dimensional cluster configurations. The simulation was extended up to the cluster fragmentation which is observed through dimer evaporation.