Since there's activity in this thread again, I might as well provide an update:
Turns out there was something wrong with the one water temperature gauge- I tried all combinations of it with another gauge and the two dash clusters I had. In all cases it was the only weak link so I ended up not using that one.
The tachometer was also fine too. Turns out my throttle cable was a bit stuck too and made me think otherwise.
Fortunately, I'm not dealing with the S4/S5 gauge swaps.
