Strange ram issue that drives me nuts

Hi Fixers,
my beloved and immortal MBP 2011 seems a little bit mortal today, I need some geek help.

Suddenly a crash. Restarted. Another one. And on and on.
It enters in Safe Mode. Disk is fine, no errors.
I have an almost dead battery, ordered here a new one.
I just thought it could be a battery and consequent system-crazyness-apple-stuff.

I had to do some task, so I disconnected the battery in order to go direct on AC.

Wait. Another crash. And on and on.

So entered Single User Recovery: no disk errors here, as it was there.

Tried with AHT: it crashed on checking the RAM.

GOT IT!

I thought. BUT. Here starts the crazyness, listen carefully.

I have 2 banks of 8gigs. With both of them I got continue crashes on boot.
…any different slot and banks combination works like a charm. So the problem is not upper or lower slot or is not bank1 or bank2 of mine.

This happen with or without the battery pack connected to the Mac.

This is the point when I need some help: could it be a dual channel problem? What can I try to do?

Another cool weirdo behaviour: as you may know when you go without battery the fans are always full speed. They are full speed only before crashing: with the single bank (1 or 2, up or down slot) fans are working as battery was plugged.

Thank you in advance for your time!
Stefano