JT, no I did not mean to say that the autopilot altitude "wandered".
Rather that as aircraft indicated airspeed increases the altitude which the autopilot maintains whilst in altitude hold mode will increase by 100-200 feet, and then remain constant.
This is nothing to do with the pitch oscillation phugoids which are another feature of the 146, particularly in icing conditions.
As they say in Woodford, UK,
"Its not broken, its British!"