You say you have water cooling, giant fans, and the rest... and old heatsink paste. I'd lay the blame at the lack of paste.
I don't know AMD chips (tried one once, had problems, went back to Intel), but if they generate that much heat, then the paste is a vital part of the installation. Without a heatsink, that chip is going to go past max operating temperature in a pretty short time, and will then shut down. No or inadequate paste = no heatsink.
I bought a laptop, years ago, at a very good price. I found out why: it would shut down after some random length of time, then come back after a while, then shut down again. The fan ran continuously. I opened it up, found NO heatsink compound, and smeared the chip with the Thermpath I use for power transistors. Problem solved, fan ran very occasionally, and laptop worked flawlessly till I upgraded. My daughter still uses it.
I use Thermpath, but I bought my pot of the stuff about 20 years ago - I don't use a lot. I'm sure it or something similar is easily available from Maplin etc. You don't need much of it, but you do need to clean off any old gunk from the chip and the heatsink face. I use isopropanol to do that.