The ultrafast dynamics of molecular nitrogen (N2) just below the ionization threshold has been investigated by time-resolved photoelectron spectroscopy using a single harmonic centered at hν = 15.38 eV. The evolution of the Rydberg wavepacket launched by the ultrashort EUV pulse is probed by a time-delayed femtosecond NIR laser pulse. The observed photoelectron spectra show two series of vibrational peaks to the ground X2Σg+ state and the first excited A2Πu state of N2+. Among these, two photoelectron peaks with the vibrational quantum numbers vX+ = 4 and vA+ = 1 exhibit clear anti-phase oscillation with a period of 300 fs, showing that two Rydberg states converging to the X2Σg+ and A2Πu ionic states interact with each other, thus causing periodic switching in the population of the ion core states.