My workaround was to add electric_always_available=1 in the [electrical] section of the aircraft.cfg. Like this:

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

I don't use it now. I guess I must be doing everything "right" when starting the 727 because I no longer lose the avionics after 15 minutes or so. ;)

Mark