Because the default adapter always stays at 99% load while the two others go down, then up, than down...
I've seen that happen with too low a voltage, if you haven't changed the voltage I'd look at trying another PSU.
I use corsair HX 850 and it powers 3x 7950's with ease.
I also have seen this happen with voltage/over-clocking issues. Sometimes there's a spot that's just unstable, how do you have the cards clocked?