Turns out you're right Cyanide, I solved the problem by purchasing FSUIPC. Looks like it's a known FS9/FSX bug.
In short, the default FS battery setting ignores/overrides the payware aircraft's power setting under certain (unknown) conditions and fizzles out in 30 minutes which knocks out the aircraft's radio and lights. Incredibly, this is the case even when the aircraft's APU/external power/engines are running with electrical buses properly configured. Engines continue to run or can even be started and all the cockpit displays function properly - only interior/exterior lights and radio are knocked out.
Registered version of FSUIPC's battery life setting (under "Miscellaneous" tab > "Additional facilities" section > click on "Extend battery life (indefinitely if value = 0, else by factor)" and set box to "0") completely solves this problem.
Previous threads on this topic:
http://forum.avsim.net/topic/213205-fsuipc-registration-required-battery-power-out-fast/
http://forum.avsim.net/topic/360732-aircraft-goes-dark-after-sitting-for-awhile/
In my case, changing the aircraft cfg's "electric_always_available=0" under [electrical] section to ""electric_always_available=1" did not solve the problem.
I have no idea what the "certain conditions" are because my posky+PSS 777 merge (sorry Andy, typo in my previous post) worked fine for many years and my PMDG 747 still hasn't experienced this issue. I even went through the painful process of reinstalling my FS9 to see if it was caused by a corrupt file but to no avail. One pattern seems to be that the problem occurs if I start the flight in areas with add-on scenery vs default scenery but I could be wrong.
Anyways, thanks for all your suggestions and replies!