First. If you do water cooling, do not under-volt, it beats the whole idea being on water. Being on Air, yes, for sure UV is a good idea, but not water, I use a shoddy Bykski waterblock that didn't even have proper screws and lacks right mounting holes, but the temps are absolutely fine. Not my first rodeo on water. Your BIOS is also tailored to eat as much it can as it designed with a large cooling budget. All our attempts to interfere with AMD boost algo in reality often brakes something and card starts to underperform.
Aren't you clogged up? The Pump RPM won't indicate a clogged loop, I saying from my own experience, just like an engine it will sound different, more strained.
My card peaks past 550W and have absolutely no problems with it. I bet there is something wrong, like thermal pad thickness for vram, so the pressure ain't enough. Try Kryosheet as last straw. It adds thickness, witch you may need. PTM79xx aint good for water cooling as usually the loop temps are under 40C so it doesn't work, it should with your soaring 100C+ tho.
Try looking how even are the surfaces of the block also. It would be great to give it another change, try taking pictures and post it here, more eyes may help finding the problem cause. For pads order some thermal putty, it will eliminate any problems with pad sizes and thickness, but only if you card is mounted in classic manner, if you use a riser and mount it vertically for the show, then putty is a no go, it drips down, we can't fight gravity.