I had a set of Echo Forged 160mm Square Taper cranks, and they did the thing which you describe. It is simply because the tapers and the ffw thread are not aligned perfectly with one another. I suggest that you forget about it and just use them, as it won't have any effect over the way the bike rides, apart from the tension in the chain, which is bloody infuriating because when the chain is on it's slack point, it takes more of a pedal revolution to tension the chain and engage the freewheel. I just ride my bike if I were you, as I did for over a year when mine did this.