We report results of the electrical resistivity of the silver salt of 2,5-dimethyl-N,N'-dicyanoquinonediimine, (DMDCNQI)2Ag, at normal and applied pressure as well as of the thermoelectric power at ambient pressure. At room temperature, pressure induces a transition towards a more conductive state. The phase diagram of this compound includes the temperatures of resistivity minima and those of the metal-insulator transition. The first are shifted to lower temperature by pressure while the second are less sensitive to applied pressure. The present data together with previous results on ESR and X-ray diffuse scattering support the idea that the ground state is spin-Peierls. The phase diagram is discussed within the framework of a theory of competing interactions, elaborated for TMTSF and TMTTF salts.