Y ELEC PUMP.........OFF
(The PTU will run if the difference between G and Y is >500psi)
APU Bleed ....ON
wait 10 secs
ENG 2.......Start
Could it be because the Y Elec Pump has been off for >10secs, there is a possibility of a drop in Y HYD PX, and then the PTU Auto test, you could get a further drop in Y HYD Px and therefore not having enough HYD Px to guarantee the NWS needs?
That's all I can think of? (We don't do this, and very rarely do OE Taxi IN, so I'm just guessing)