There is a simple fix for this.

Open the aircraft.cfg, which can be found in the FSX\SimObjects\Airplanes\CS_B727-100 folder, with notepad and add electric_always_available=1 in the [electrical] section.

Like this:
[electrical]
electric_always_available=1
max_battery_voltage = 24.0
generator_alternator_voltage = 30.0
max_generator_alternator_amps = 400.0

This should prevent the avionics from switching off. My thoughts are that you are missing something in the procedures that is causing it to happen. I used to have to have that line in mine for the same reason. I don't need to now and I always have power in my avionics.

Hope this helps.

Mark