Voltage Drop off completely?

Hi All again,
I have been recently been having an electrical problem, apologies for the length;
Intermittent symptoms;
• When pressing on the brake, it seems almost like the battery completely gets switched off then on again.
• When this happens I can get a Gearbox Fault and then stuck in top gear.
• It has also killed the engine at times, possibly due to the gearbox jumping into the higher gear when at slower speeds.
• Flashed high beams today and gearbox faulted and killed engine as well, this is fun out in traffic!
You get the idea.

The Voltage tests results as per the sticky;

1. Engine off, all accessories off prior to other tests. What voltage 13.17
1a. Engine off, all accessories on for 30 seconds, then off. What voltage DID’T DO THIS ONE, sorry.
2. Voltage across battery at idle (no electrical load) 14.25
3. Voltage across battery at 2000rpm (no electrical load) 14.26
4. Voltage across battery at 2000rpm (everything electrical switched on) 13.45
5. Voltage at ALTERNATOR at 2000rpm (everything switched on) 14.33
6. Voltage drop between alternator body and battery negative (YES negative). Measure at max electrical load and 2000rpm 605 (millivolts please)
7. Voltage drop between alternator positive and battery positive. 2000rpm, max electrical load 270

When doing test 4, it seemed like the car was having a heart attack, but then came back up to normal power& revs. Didn’t fault though (was having a good moment).

My assumptions are;
• Battery still good, recently replaced.
• Alternator seem to be charging /or outputting correct voltage.
• Bad leads.

Would this be causing what is happening when I draw more power (like brakes and lights etc.) and effectively shutting down the ecu’s perhaps??

Thanks again in advance,
Tyrone