  We report on long-term monitoring of the anomalous X-ray pulsar 1E
1048.1-5937 using the Rossi X-ray Timing Explorer. This pulsar's timing
behavior is different from that of other AXPs. In particular, the pulsar shows
significant deviations from simple spin-down such that phase-coherent timing
has not been possible over time spans longer than a few months. We show that in
spite of the rotational irregularities, the pulsar exhibits neither pulse
profile changes nor large pulsed flux variations. We discuss the implications
of our results for AXP models. We suggest that 1E 1048.1-5937 may be a
transition object between the soft gamma-ray repeater and AXP populations, and
the AXP most likely to one day undergo an outburst.
