  We report measurements of thermopower oscillations $vs$ magnetic field in a
diffusive Andreev interferometer. Upon the increase of the dc current applied
to the heater electrodes, the amplitude of these oscillations first increases
then goes to zero as one would expect. Surprisingly, the oscillations reappear
at yet higher heater currents with their phase being $\pi$-shifted compared to
low current values. From direct measurements of the temperature gradient we
estimate the amplitude of the oscillations to be orders of magnitude smaller
than predicted by theory.
