That *is* exceedingly strange. Are you sure it's the Epson and not (say) the power strip that they're both plugged in to? What about the serial cable from the Epson to the Mac? Is it flaky?
Any event, the power supplies in the PowerComputing machines are known to have problems. I had to replace the one in my PowerCenter. It manifested itself in a much less strange way...it wouldn't turn on unless I used the power switch on the back (keyboard power wouldn't work) and sometimes it would spontaneously turn off. So I replaced it with one from PC Power & Cooling
. The PowerCenters (and, AFAIK, the PowerTowers) just used standard ATX power supplies. Since then it's been rock solid.
The powerwatch site is a great place to check.