  We present an instability, occurring in the inner region of magnetized
accretion disks, which seems to be a good candidate to explain the
low-frequency QPO observed in many X-ray binaries. We then briefly show how, in
the remarkable case of the microquasar GRS 1915+105, identifying this QPO with
our instability leads to a scenario for the $\sim$ 30 mn cycles of this source.
In this scenario the cycles are controlled by the build-up of magnetic flux in
the disk.
