As someone without a steak in this I have 4 guesses as to what might be wrong.
1) A Bad mount. Either with the mounting pressure or how the thermal paste was applied. This would show up as a sharp temperature increase when put under load.
2) The pump/fan speed is too low. This would show up as a slow build as the water heats up.
3) The water is flowing the wrong way through the block. This will have a stable temp about 10C higher than you expect.
4) Air bubble(s) in the block. This can have a similar temp profile to the bad mount.
I hope you get this figured out. I only want to know what was wrong out of academic curiosity.