2013 XC70 Odd Blower Issue

[DamoV70T5]

The blower motor on my HVAC system used to work fine. Now it works fine when the ignition is on but the engine is not running (good air flow from the vents, fan runs slow to fast when the settings are varied as it should etc). But when I start the engine the airflow drops dramatically and the blower fan will only run at slow speed even though the console indicates that I have turned it up to max.

Thoughts appreciated.

Damo