Dear Geekzone members

Following used system purchased 2019, without drives.

Apple Mac Pro 5.1 2010 6 Core 3.33GHz 16GB RAM (5.1, not a flashed 4.1)
ATI Radeon HD5870 GPU
Apple Cinema Display 30" monitor with Apple Mac power adapter

Sourced drive parts following, all used, and assembled.

Angelbird Wings PX1 M.2 NVMe PCIe SSD Adapter
Samsung MZHOV512HDGL SSD OS drive
Seagate Barracuda 1TB SATA HD storage drive

Split SSD into two volumes, Apple Mac OS X 10.6.8 on one volume / no OS on other volume.  Loaded OS X 10.6.8 Snow Leopard from original Apple CD.

Miraculously for this rookie, everything worked.  Happiness!  Nice big monitor, SSD much faster than HD based operating systems I had used previously.

Then, some time later, weeks / months, Apple white iMac plastic keyboard stopped working from Apple Cinema Display powered USB port.  Works fine from Mac Pro powered USB ports.

Then, some time later, weeks / months, Cinema Display would flicker immediately after boot.  Would settle ten minutes, then flicker again, then finally settle with protracted use.  Also Cinema Display would not respond to Apple Magic Mouse or keyboard to wake from sleep.  These Cinema Display issues can be resolved by unplugging power lead from Cinema Display power adapter momentarily, then plugging back in.

Then some time later, weeks / months, Mac Pro developed booting issues, which is where things are at now.  Turn power switch on, light illuminates, optical drive engages, fan starts, PCIe SSD adapter illuminates, HD noise may or may not occur, no boot chime, hard reset.  Usually have to go through this cycle anywhere from 6-12 times before boot chime sounds and OS login appears on Cinema Display.  Which then goes through its flickering regime.  If machine left on overnight, sometimes it will wake for login, other times it will switch off overnight and require rebooting.  Occasionally, certain tasks will cause the Cinema Display to switch off.  Once it boots the Mac Pro itself is fine, but the Cinema Display will be unstable until ten-fifteen minutes have elapsed.

There is a good machine hiding in here, but I suspect there are hardware issues at play.  I have the Mac Pro 5.1 technician repair manual, but it starts getting a bit invasive for my capabilities so keen to get any clues of common hardware faults with this machine.

PRAM and SMC resets have no effect.  The battery has been changed.  Dust has been cleaned out.  Disc repair performed, no issues.  OS X reloaded, no change.

Thanks in advance for any inspired advice.

Regards
SJM