Hi everyone!
I have this silly problem

I have a Diamante ES 2005 3.5V6 (Australian made)
Recently it started doing this
Battery dies all the sudden, last time it did it after just 6 hours from parking it. Had a mechanic look at it, tested the altenator said all fine. after starting the car it keeps turning off every time you brake (stop or slow down) battery light comes on and the oil light (mostly battery then when it shuts down the oil light comes on). Was told it was the battery and that it doesnt hold charge. I just bought a brand new battery heavy duty (600cc) was driving out of the shop and ooops it did it again.
Now it only does it for about 5 to 20min and then it goes fine, thats why the first thought that the battery is not holding the charge.

Also upon starting the car it goes fine idles fine, then you move, and on first brake it starts acting up, and keeps doing so for about 5-20 min then goes fine.

Any ideas on what is causing this? cant trust the mechanics here as most of them dont have enough knowledge, or they are simply a rip off, so need to know what is causing this and not get ripped off.

If you need any info please let me know.