Go to FSX\SimObjects\CS_B727-100\ and open the aircraft.cfg file. Search for [electrical], and when you find it, add the line electric_always_available=1 to it. Like this:

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

That will fix the problem. You'll need to do this for the -200/100F/200F as well, if you have them.

I had to do that too, at first, but once I learned how to set things the way they need to be for flying, I don't lose the avionics anymore and no longer use that line in my CS727. It was Lou who help me get things right.

This is where I asked Lou about it: http://www.captainsim.org/yabb2/YaBB.pl?num=1267897624/13#13.
This is where Lou responded and explained how to do it right: http://www.captainsim.org/yabb2/YaBB.pl?num=1267897624/15#15

Before using that fix, I used to press CTRL+E (engine start) and it would bring back my Avionics. It would not start the engines if they were already started.

Mark