Sir fenner . . . .
With the unit plugged into the laptop, one could expect max power to the fan from either the lap top battery OR the Lap tops external POWER BRICK.
When you are plugging into that other usb adaptor supply I am suspicioning its current capability output level to be at a max of the charge current supplied for charging an external device.
This is probably being LESS than your fan is needing and the voltage is pulling down and the converter is trying to compensate and bring up the voltage, but it is finding that it can't provide that additional power, so the unit is slow cycling in that attempt to correct.
You say . . . .
a usb adapter from the 13amp mains socket with a voltage of 5.10v
The question there, is if you measured that voltage under fan load, if not, you need to keep your DC metering continually monitoring that " 5.1 " volts to see if it STAYS at that level . . . . or if you find it to be pulling down and then rising in unison in the same manner that " the fan intermittently jerks round ".
If so, you might attempt to shunt a 1 thru 2000 ufd @ 12VDC electrolytic cap across the 5VDC USB adapter output to the fan to see if that will let it enhance its power capability slightly, on upward to your fans needs.
73's de Edd
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