The isolated neutron star (INS) RX J1308.6+2127 has been observed with Chandra and XMM-Newton several times between 2000 and 2007. The six most recent XMM-Newton observations are part of a monitoring campaign we initiated to study the long-term temporal and spectral properties of INSs. The primary goal of this investigation is to better quantify the similarities and differences between INSs and magnetar candidates (i.e. Anomalous X-ray Pulsars and Soft Gamma Repeaters) in terms of their spectral and temporal variability. Here, we present our analysis of the current data set and show that (i) the energy spectrum is well modeled by a blackbody with two absorption lines, (ii) the spectral parameters do not show significant time variability over the last seven years, (iii) the pulse profile is strongly energy dependent, but stationary, and (iv) the pulse frequency evolution is consistent with monotonic spin-down. Our spectral and temporal results are consistent with earlier analyses by Schwope et al.  and Kaplan and van Kerkwijk , respectively. The inclusion of the new monitoring data provides more precise constraints on both spectral and temporal parameters of this INS. For isolated neutron stars whose X-ray emission is not powered by spin-down (INSs, AXPs and SGRs), RX J1308.6+2127 is among the most stable members.