Ok so i run my 8pac 3200mhz memory at 4000mhz C19-19-19-39. If i change it to C18-19-19-19-39 my cpu score on 3d mark drops from 8865 down to 6641 (8700k at 5ghz). I was just wondered if anyone knew why as its a huge drop, the system is still stable.
https://www.3dmark.com/compare/spy/6169803/spy/6169713
That is a huge drop for such a small change, but remember that overclocking your RAM to anything above the rated frequency of 3200Mhz, and then tweaking the mem timings, the mem timings will have a greater effect on stability, the higher frequency you go.
Something isn't right with that C18 from the C19, causing (just repeating what others have stated here) additional memory error in the reading different memory addresses, the CPU will then try to rectify these errors but it takes additional CPU cycles to fix errors, so your overall score goes down.
I would tweak more than just the C18-19-19-19..try 18-18-18-18 or play around with the '39' at the end, dropping it by 1, or increasing it by 1-2.
Also, if you want to see if you are producing more memory errors while benchmarking, opel up Task Manager, under Performance and click on Open Resource Monitor then go to the Memory option & on the right you have standard Task Manager graphs/timelines, there is one that shows how many 'faults/sec' you're getting (if I remember this rightly).
Check the difference in number of memory errors produced from your previous OC to this one.
Alternatively, if you really want to guarnatee full stability, Memtest for 6-8 hours (or minimum 2 passes, ideally 3 = 10-12hours-ish or more) and you will know for sure, how bad that one little timing change actually was (and how stable your system actually is).