   Fuel contamination is the leading suspect in the problems,
which Boeing's message described as "uncommanded engine
oscillations."  In most cases the instability disappeared after
"a few seconds to a couple of minutes," but two incidents last
year suggest the potential for a more serious problem.
Alaska Airlines declined to comment and referred questions to
Boeing.     Engineers haven't yet pinned down the cause,
although Boeing suspects it may be something in the jet fuel.
"These events are highly localized phenomena. All events have
occurred on airplanes that were recently routed through western
United States airports," Boeing's message to airlines said. "The
investigation has narrowed the leading root cause to
intermittent fuel contamination."    Nevertheless, the pilot
returned to the airport.    "We've increased the focus on it
since last year," he said.   "The engine, the fuel control unit,
the fuel, it's all part of a system," said Tull. "We're looking
at the whole system."  "These are extreme