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Interestingly, here is an HP Z420 Workstation User with 4x32GBSamsung M386B4G70DM0-CMA3 RAM, running at 1833 MHz:
Link: HP Z420 Workstation Performance Results - UserBenchmark.
Perhaps you can try out 4 x 32GB (in HP's designated RAM slots) and see if that would make a difference?
Anyway, the best answer I can give you is that not all RAM (for an HP Z420 at least) is created equal.
When you look at these HP System Z420 Workstation Users, you'll see plenty of RAM running at 1600 MHz.
However, based on the aforementioned User benchmarks, these are the RAM that consistently will run (well, should) at 1833 MHz:
Samsung M386B4G70DM0-CMA3 4x32GB
Samsung M393B2G70QH0-CMA 8x16GB
Samsung M393B2G70DB0-CMA 4x16GB
Samsung M391B1G73QH0-CMA 4x8GB
Micron 18JDF1G72PDZ-1G9E 8x8GB
Micron 9JSF51272AZ-1G9E2 8x4GB
Hynix HMT41GU7BFR8C-RD 4x8GB
Hynix HMT41GR7AFR8C-RD 8x8GB
Mind you, this is not an exhaustive listing, but it will give you an idea.
Kind Regards,
NonSequitur777