Thanks AAC47, that's interesting. The flight manual states:
"For all jet aircraft flying at high altitude, the damping of
the long perlod oscillations is rather weak, causing speed
and altitude variations which may result in difficulty in
maintaining an accurate flight path, Although the aircraft
is quite stable under ail flying conditions, the autopilot
and the altitude control should be used for optimum performance."
Next time I encounter oscillations in level flight I'll try your technique of going to manual and retrimming. Pretty sure I tried this last time but the oscillations returned after a few seconds upon autopilot re-engagement. They weren't severe movements, about 50-100ft above and below the target. I did consider descending to a lower altitude but on that flight I wanted the better the fuel consumption at the higher flight level so I stuck with it.