Not so fast. If the supply voltage is marginal, then some logic boards may work, and others may not. The only way to preclude power supply problems is to make an actual measurement.
Also, if you put the 128K board back in, does the problem recur? Many power supply problems turn out to be connector problems, which the simple act of disconnecting and reconnecting can fix (temporarily, at least).
That said, you probably do have a logic board problem (bad RAM is more and more likely to be the correct diagnosis, given what you've found, and given the quality of US DRAM at that time). But check the other possibilities first.
Also, if you put the 128K board back in, does the problem recur? Many power supply problems turn out to be connector problems, which the simple act of disconnecting and reconnecting can fix (temporarily, at least).
That said, you probably do have a logic board problem (bad RAM is more and more likely to be the correct diagnosis, given what you've found, and given the quality of US DRAM at that time). But check the other possibilities first.