  The large majority of neutron stars (NSs) in low mass X-ray binaries (LMXBs)
have never shown detectable pulsations despite several decades of intense
monitoring. The reason for this remains an unsolved problem that hampers our
ability to measure the spin frequency of most accreting NSs. The accreting
millisecond X-ray pulsar (AMXP) HETE J1900.1--2455 is an intermittent pulsar
that exhibited pulsations at about 377 Hz for the first 2 months and then
turned in a non-pulsating source. Understanding why this happened might help to
understand why most LMXBs do not pulsate. We present a 7 year long coherent
timing analysis of data taken with the Rossi X-ray Timing Explorer. We discover
new sporadic pulsations that are detected on a baseline of about 2.5 years. We
find that the pulse phases anti-correlate with the X-ray flux as previously
discovered in other AMXPs. We place stringent upper limits of 0.05% rms on the
pulsed fraction when pulsations are not detected and identify an enigmatic
pulse phase drift of ~180 degrees in coincidence with the first disappearance
of pulsations. Thanks to the new pulsations we measure a long term spin
frequency derivative whose strength decays exponentially with time. We
interpret this phenomenon as evidence of magnetic field burial.
