fault code P0560 system voltage malfunction (alternator overcharging)

Hi, I have a 51 plate vauxhall corsa 1.8, ive had it since last september, around january i had problems with my batteries dying overnight, i wasn't driving but i was starting it a few days a week, but i replaced the battery twice and it kept happening, so i had to keep jump starting it to allow the engine to run for a few minutes.
at the time when the battery was about to go all the electronics inside the car would switch off (electronic power steering, head unit, the speedo would freeze etc.

i decided to do a fault code read on it and it came out with P0560 system voltage malfunction, and i tested the voltage output on the terminals and it had a reading of 17-19v i decided to change my alternator and auxiliary belt and the problem went away for about a month until i was driving, just as i turnt out of my area all the electronics on the inside switched off again including the EPS so i checked the voltage and it came out around 17-19v again and the same code came back, I'm not sure what to do at the moment? could it be one of the earth points around the car?

When i put my AC on full the voltage goes down to 13.5v which is should be? but what is causing this? when i drive the car when the revs get to a certain point around 2-3k everything cuts out again? HELPPPP?