I have a 12 x 1070 rig too with 2 x 1250w psu they are running fine.  Most likely the problem is with the risers (that includes cable to check too)  and you can try reinstalling driver / cuda to save you some time, just in case its a driver problem.

have you used the PCIe x16 port on MB when plugging in your risers or are all of them plugged in small x1 ports?

Say what? How on earth do you come to that conclusion? Lets see your math. Ill show you mine right now.

Rig with 12 1070Ti's at 150 TDP each. (No overclock or TDP adjustments) https://www.buriedone.com/hashrates

6 1070 TI's per PSU= 6x150w=900w

PSU1= 900w
PSU2=900 + CPU/Motherboard/SATA 200w= 1100w

and each of his PSU's are rated for 1300w

I would still give them a 65% TDP Overclock  +150 clock/+500 memory and the power usage will go even lower. https://www.buriedone.com/tuning

Problems are probably a combination of BIOS setting and/or update AND/OR Driver issues during install.  Go into device manager and click on view tab then show hidden devices and have a look at the mess under Display Adapters and Sound and Video Controllers.

DDU to remove it all and disconnect all GPU's.  Install one at a time and reboot after drivers install for each GPU.  Boot back up and verify GPU installed successfully.  Then and only then shut down and install next GPU. Repeat cycle until all are installed.

actually TDP on my 1070Ti's and P104-100's is 180W, bro. loading your PSUs to the limit isn't good thing either, better to have some power reserve remaining.

if I understand OP right - 1070's run fine, but there are issues with 2x 12x1070ti's rigs. I am pretty sure this is related to additional power that Ti's require.

Say what? How on earth do you come to that conclusion? Lets see your math. Ill show you mine right now.

Rig with 12 1070Ti's at 150 TDP each. (No overclock or TDP adjustments) https://www.buriedone.com/hashrates

6 1070 TI's per PSU= 6x150w=900w

PSU1= 900w
PSU2=900 + CPU/Motherboard/SATA 200w= 1100w

and each of his PSU's are rated for 1300w

I would still give them a 65% TDP Overclock  +150 clock/+500 memory and the power usage will go even lower. https://www.buriedone.com/tuning

Problems are probably a combination of BIOS setting and/or update AND/OR Driver issues during install.  Go into device manager and click on view tab then show hidden devices and have a look at the mess under Display Adapters and Sound and Video Controllers.

DDU to remove it all and disconnect all GPU's.  Install one at a time and reboot after drivers install for each GPU.  Boot back up and verify GPU installed successfully.  Then and only then shut down and install next GPU. Repeat cycle until all are installed.

8 GB of RAM is enough. I am in fact running the same 12-GPU rig of 1070s as yours using only 4 GB of RAM.

My suggestion to you is as follows:

1. Reset your overclock settings. You might be overclocking one or two GPUs in the 2nd row beyond their capacity. Not all GPUs are created equal.
2. Find and isolate the problematic GPU. Try to start adding GPU's from your 2nd row one by one and start mining. See which GPU causes it to fail.
3. Try running multiple instances of your miner. Copy paste your batch file and run 6 GPUs on the first mining instance then do the same with the remaining.