Dashboard lights flash and all gauges drop for a second then back to normal

[Nesf1]

Hi! I have a 07 Jeep Liberty. While driving, every few minutes, I've been experiencing a few dash lights come on for a second (ESP, ABS, Traction and Brake) together with the gauges starting to drop, then all goes back to normal, while no known impact on car's performance. My headlights, interior lights and odometer reading light have a light flickering. Gas gauge shakes sometimes too. I suspect I have a bad ground, but where? I did a dynamic battery voltage test, it reads 12.58, however, when I remove the negative cable and put it on any other ground (engine, ground at firewall, ground near the battery) they all show an unstable reading that moves between 12.8 and 13.6. The battery and alternator are brand new.

 

[KJowner]

Are you measuring the voltage with the battery cable disconnected and the engine running? If so the voltage will be all over the place without the battery to stabilise it, you risk damaging the alternator and car electronics doing that!

 

[Nesf1]

No, both battery terminals are connected while I'm testing it. When I mentioned "when I remove the negative cable and put it on any other ground", I meant multi meter cable.

 

[KJowner]

Try putting a jump lead between the battery and a good clean ground point