I am posting this for my brother inlaw, since I have used this forum and with great results. He has a 98 bonniville SE with 3800. He has an issue of it dying randomly. When it dies he has no power to anything but the dome lights and hazards. No power to windows, locks, seat. He has a good mechanic but I think he is grasping at straws now. They believe it is the security system. They replaced the ignition and bypassed the security box under the dash. the wierd thing is, the last 2 times is has died, he gets in it and moves the power seat button back and forth until the seat starts to move. then the car will start. I was thinking maybe a short some where. I will have to relay any info to him because he does not have internet. Thank you for your time.

If it was a security issue, while the engine was running, you would get a security light. If not, the engine would crank, but not start.

Sounds like this might be the issue...
Ground bus

Let us know how it looks.

I'm completely with Mike on this one.

In 98 the security system can only disable the motor from starting by turing off the fuel injectors after it primes them for start up. Odd I know. So if your security system is complaining the car will fire and die. Fire and die..fire and die. The system will not turn the car off once it is running.

This very much sounds like a little moisture under the carpet etc.

Thank you both for your input. I am having him check out the ground bus and get back with me. Thank yo for being patient.