Is it me, or did you get it stable before on the other thread? I've gone and read through both of them. Did you do any silly runs at anything more than 1.4v for the Vcore? If I remember didn't rich kill his 980x by pushing 1.4 or more vcore on it?
Could, in the process of running the PC at something higher than its been specified to, and without research being done on the motherboard and chip early on in the OC'ing process, that you might have damaged something.
Not all chips clock like each other, not all of them clock very much at all.
Could it be that to get it running stable at stock, that a slight increase in voltage might be needed, either to the motherboard, cpu or memory?
Just read up on overclocking with the Ud5 and take your time overclocking like other people on the forum have rightly said, if an expert, like the staff at OCUK have said be patient, be patient and clock it steadily, and note down all the voltages that you've used to attain the speed that you want. Just because someone has clocked it at 4.3 doesn't mean that yours can be stable at 4.3.