Modem powercycling under load

[The Face]

     Ok, so a while back, my modem (Sagemcom F@st 2864) began to powercycle whenever I put it under under any significant load (ie. game downloads via steam or P2P sharing). It would be fine for about 2 minutes, then powercycle/reset itself (taking about 5 minutes), wash rinse repeat until what ever was downloading finished. This was a modem that I owned (and currently do) but I have since swapped it out for the same make/model (which works just fine). Before replacing the modem I fixed a damaged power cable that was causing what was probably a brownout situation. After I replaced the cord, the problem persisted however the new modem is currently operating with the exact same ethernet and power cables as the old one (and working fine).

 

     I'm looking to fix the one that powercycles so I can sell it and recoup some of the cost of replacing it. What I hope is that it's not is a hardware failure because I'm not an elecrical engineer. Does anone have any ideas?