With a 97 ls400, press the brakes, and everything shuts down.

I would first concentrate on all ground cables, straps etc going to firewall, battery, engine etc.

IS the battery original spec? the amount of electrics on these cars is huge and hence the original battery being the size of a trucks. LS does take a 27F Battery and it is 710 CCA

Check boot lights are switched off.
Ensure you lock the vehicle when leaving it too, this activates "Sleep mode" and the car shuts everything down.

have a diagnostic done,, I'm thinking its the MAP sensor, but the only way to be certain if from a diagnostic..

DUDE!!! Installing a new ground Worked for these symptoms ive been having for MONTHS! Dim headlights, pressing brake would shut off radio, turn signal slow and non blinking, also would shut down radio, dash lights dim and gauges FREAKING OUT!!! Process CHEAP AND EASY!

if you don't press the brakes will it still shut off:? i would check to see if the brake booster is bad , the most simple way of checking this is to clamp off the vacume hose going to it and press the pedal if the vehicle shuts off you have a different problem if it stays running most likely the booster is punctured internally causing the engine to have an abrupt lean condition and shut off

There is a problem with the brake switch. look under the dash at the brake pedal. follow the arm the pedal is attached to and about 8 - 10 inches above it is a box with a botton that is pressed when the brake pedal is all the way back(off). in other words, when you let off the brakes, the button is pressed which turns off the brake lights. see if that box has moved and if the pedal is still pushing that button on it in far enough to shut off the brake lights. try pulling the brake pedal back by hand and see if they shut off. Hope that helps.

You will have to go to Advance Auto Parts, or Auto Zone, and they will scan your computer for free and let you know whats going on. From what you are saying your vehicle may be in "limp" mode which begins to shut down certain functions to maintain stability such as governed speed, fuel consumption etc.

Faulty ignition switch. Replace

I'd rechagre your battery fully then test the alternator. It sound to me like you have a bad alternator.