Thanks! No, it's definitely not a power issue, I had it hooked up to a AX1200 and a AX1200i. I'll test that out this weekend.
Note that ATX PSUs are supposed to have separate "rails", each with over current protection. Thus a larger PSU has more "rails" (PCI-E) connectors but each has separate over current protection.
Adding another scenario:
a) The relay closes
b) The PSU over current protection drops the voltage.
c) he Cube sees the voltage drop and opens the relay.
d) The PSU no longer has an over current condition, restores the voltage.
e) The Cube sees sufficient voltage, closes the relay
This repeats and the relay buzzes.
I would try adding a pair of large caps (10,000 uf) to the PCIe connectors as per the earlier parts of this thread,if it is voltage drop condition and it is only while the cube is starting then that may fix the problem, if, however the PSU cannot supply the continuous voltage necessary then that will make no difference.