jremington:
Motors and servos all draw the stall current every time they start up.
I didn't know that, though now you've told me it seems obvious. A good bit of knowledge to bear in mind for the future, thank you. I'm not convinced that that's the problem here though.
I've found a site ( http://www.arxterra.com/micro-servo-tower-pro-mg90s-current-test/ ) that claims that when shifting 500g @ 1.6cm, the mg90s only draws 460ma. Is "stall current" really likely to be over an amp? In which case, how is it running the project's "propulsion motor" successfully when that's a motor size up from the one in the servo? The 5v cooling fan I tested with is a much larger motor than either the servo or propulsion motor, and it runs fine.