Intermittently not starting & not cranking, doing my head in

[Rosato7]

My partners 2012 KA is being a complete nightmare!

Firstly, it started (sometimes) juddering and wanting to stall as she was driving her commute.  One time the power-steering stopped while she was driving.  I could never get it to replicate these issues while I was driving it.  
Then one day the car wouldn't start and she had to get a lift to work, I tried the car later that day and it started fine! (partner not impressed) 🙂

However, that past week or so the car really has been out of action.  Put the key in the ignition and turn to position 2 and get warning messages sto/start unavailable, ABS unavailable, Fuel-cut off unavailable. and car will not crank/turnover at all.

I've checked the battery with multi-meter and showing 12.2/12.4.  I've searched around this and fiat 500 forums and discovered the "Bad-Earth" fault issues, which after testing, looked like it could be.  So, I've replaced the rotten earth strap and now car is showing same readings across the engine, as at the battery terminals.  BUT it still wouldn't start under it's own power!

But here is the WEIRD thing.....I put a battery pack on it and managed to start it!  It struggled to idle, had to rev the hell out of the engine to keep her going and the REV's kept dropping by ALOT initially (As my foot was on the accelerator).  But after a few minutes it calmed down and could hold the idle itself.  Then I could start it again off its own battery.  I am totally perplexed by what is going on here....has anyone got any suggestions at all??? 
 

[unofix]

This is one of the rare occasions when I recommend a brand new fully charged battery.

If you are seeing battery terminal voltages as low as 12.2V then it's time to replace it.