I have a theory......when the consistency of the coolant turns to slush and the evaporator is all covered in thick slush.....it no longer works very well to cool the coolant.
OR.....the coolant, once it turns to slush cools much slower. Like if you have a freezer full of water bottles....when they freeze up today they will not be as cold as the will tomorrow if left in the freezer. Ice cools slowly.
OR.....my chiller won't take anything below -31 C anymore, when last week it took some coolant down to -45 C. All I have left to thin it with is that jet fuel I got the other day mixed with antifreeze.
I'll try a gallon or so and see if it helps tonights chilling.
EDIT: here is a picture of the slightly chilled gpu loop.