Power supply of spindle HF500 dies for a second time in 1 week

Hi,

I have a D600 with HF500 spindle for several years but not frequently used. Working on a project, last week the power supply died while making a sample/test model in MDF.
I used a 3mm 2-flute cutter at 10.000 rpm, advancing 600mm/min and 1 mm per pass. 
I replaced the internal power supply of 24V/20A with a new one fully compatible in electrical and physical dimensions. 
Today, I was making the same project but in walnut and again, after about 3/4 hour cutting, the power supply died again. Same settings, same gcode, ...
Note, that spindle was cooled with the Hailea ACO-208 Air Pump as originally delivered.

Any idea what I might need to do to overcome this in the future?  What could be the reason for this failure? 

Thx in advance

Marc Engrie