  We report the detection by the Gamma Ray Burst Monitor onboard BeppoSAX of
the strongest and longest ever detected outburst from SGR 1900+14. Oscillations
are detectable with a period of ~5.16 s for the entire duration of the event
~300 s. The temporal analysis reveals also a remarkable periodic substructure:
after about 35 s from the event onset each 5.16-s pulse shows a pattern of four
subpulses and a dip, each separated by ~1 s. Significant spectral variation is
detected during the event and for each individual oscillation. The first and
most intense part of the outburst is quite hard, and similar to what previously
detected from the `March 5th event'. A hard non-thermal spectral component
persists for ~200 s. SGR 1900+14 was proposed to be a strongly magnetized
neutron star (B>10^14 G) undergoing violent instabilities by internal
magnetic/crustal stresses. However, the onset of an apparent 1-s periodicity
within the 5.16-s pulsations and the observed spectral properties show a
complex behaviour that is not satisfactorily modelled yet.
