Interesting. It sounded like what we get when we’re trying to max out our overclock so I figured it was something along these lines.
Water cooling should keep you in good temp ranges if you’re not doing anything crazy with the clock. Maybe there is a problem with the thermal coupling between the CPU and the heatsink where the tubes attach. And the cooling system still needs good airflow (unless it has a refrigeration unit, which most do not). What kind of temps are being reported?
Getting a new CPU would obviate the need to fix it but when you take out the old one, see if the goop looks OK. And make sure you couple the new CPU well. Good luck.