Hmm, have you tried to reapply the paste and reseat the cooler again? (i know you did with the first one, I mean since making the changes)
Also, have you tried turning the radiator the other way around so that the tubes are at the bottom (I know you won't be able to keep it like that without changing the tubing, but it is good to try for testing purposes?
The 9900k is a hot chip, but a 360 radiator should be able to keep it under control, maybe not under full load for extended periods of time, but still shouldn't be peaking like this.