Won't Start, Kills All Power Temporarily - 00 Launch

I started to winterize my 2000 Launch (Indmar MPEFI 5.7) today. Got to the point where I start the motor on the garden hose to run Sta-bil through the motor, and it won't turn over.

Here's what happens:
I turn on the main power switch above the ignition switch, and the radio comes on. Turn the key to 'on', and the gauges all come up and the fuel pump cycles. Try to crank the engine, but there's just a loud click somewhere on the back by the PCM, and everything goes dead including the radio and dash.
If I turn everything off for a few minutes, it will do the same thing again.

It's as if some relay is tripping and self-resetting after a few minutes. The 50A relay by the PCM is not tripping. I've cleaned the battery terminals on both ends, and have 12.5V at the starter solenoid. Battery is an Interstate, and only a few months old.

Any thoughts would be much appreciated.

That sounds like a dead battery. I just replaced a battery that was outputting 12.3v but would not turn over the engine. On the flip side, I had a bad connection on my battery and was only getting 10.4v at the starter but the starter would still turn the engine over.

Tracetrimble, were you able to trace the problem?

You mentioned the solenoid, but have you checked tightness of the cables on the starter and the ground (i.e., where both cables land when they leave the battery)? You have to pull on them to confirm, as vibration can loosen them, causing them to mimic a bad battery.

Of course, it could just be your battery, as wise h20-ski suggested.
MySupra