You mean you loose the power even if the APU Gen is on, or the external power?  :o Does that battery trick help, from the 727? I didn't run into that problem so far, but I'm running the FSUIPC battery tweak.
FSX models a way too weak battery for all planes, maybe we're seeing this FSX flaw in action. FSUIPC can help or the mentioned tweak from the 727.

Here's the 727 tweak. http://www.captainsim.org/yabb2/YaBB.pl?num=1315183931/1#1

Here's what FSX thinks is a plane's battery. http://www.public-domain-photos.com/free-cliparts-1-big/electronics/battery/battery_jean-victor_bali_01.png