First off, welcome to ProCooling.
Your problem is obvious: you can't use a res to cool down a system.
You have to remember that the heat that goes into the system, from the CPU has to exit somewhere. In your case, it exits where the surface of your res contacts the ambient air.
That being said, the warmer the water is, the more heat will be released into the air. If you think about it, there's no heat transfer when both the water and the ambient air are at the same temp.
In your particular case, you've got good temps at idle, but because the heat transfered is much higher when the CPU is really given a run, your water temps will rise, up to the point where the amount of heat that's transfered to the ambient air is the same as what the CPU is putting in the loop.
The solution here is to replace your res with a radiator. We mostly use car heatercores, because they're cheap and efficient.
With that, you'll bring your "load" temps back down to what you currently have for idle temps, and probably even lower!
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