Hate to admit this, but I've just tried a matched pair of (separate) 300uH inductors and it seems a lot happier up to 4 amps: but I can't check the waveforms when I'm burning my finders with load resistors - other than thinking they looked OK for the second or two I could see them - I need to set up a suitable variable load to test more fully (big transistor on a big heatsink)....
OK - with the separate inductors it seems to be stable and quiet right up to 10amps
.
At low powers there's still an asymmetry in the pulses but it seems to disappear at higher currents (or at least diminish to the point where I can't see it on my dodgy scope).
Question is: should I worry about it if the PSU is otherwise working OK?