  We report a study of spin waves in hole-doped ferromagnetic La7/8Sr1/8MnO3,
in the metallic state (165K) below TC (181K) and through the puzzling
metal-insulator transition which occurs at TO'O''=159K. They reveal very
unusual excitations. Propagating spin waves are observed in the small q-range
up to q=0.25 (lambda=4a), and, beyond, four dispersionless levels. Both types
of excitations have a quasi two-dimensional (2D) character. The transition is
revealed by a folding of the dispersed magnon branch at q=1/8. In the metallic
state, the dispersionless levels reveal ferromagnetic domains with 4 lattice
spacings for their size along a and b. They lead to a picture of charge
segregation with hole-poor domains surrounded with hole-rich paths. Within this
description, the transition appears as the ordering of domains, which can be
interpreted in terms of a 2D superstructure of orthogonal stripes.
