This could be many things:
a) Deterioration of TIM
b) Dust buildup on components
c) Decrease in pump flow
d) Ambient temnps have changed
e) Galvanic corrosion
As for the last .... every liquid cooling system is subject to galvanic corrosion ... in your car, your ant-freeze has corrosion inhibitors which combat this for a time and the reason we change our coolant or a maintenance schedule is that these inhibitors have a working life of from 6 - 24 months depending on specific formulations used and the degree of galvanic activity to which they are exposed.
The level of galvanic activity is dependent upon the difference in "potential" between the metals in the loop. When the components of the loop are made from similar metals, the potential is very low. When metals are "mixed", potential is high. Boaters are very familiar with this concept and corrosion is readily observed, more so in salt water, as the galvanic cell sacrifices elections on the lesser noble metal to the higher... so a "sacrificial anode" (aka 'zinc') is used and "gives it's life" to save the propeller.
Knowledgeable users building "custom loops" will always make sure not to used "mixed metals" as this is what happens when you say use a cheap aluminum radiator and a copper waterblock. Martinsliquidlab "Busting out the science behind liquid cooling!" has long been a 'go to' source on such topics
https://martinsliquidlab.wordpress.com/2012/01/24/corrosion-explored/
You can read the article but as the saying goes about a picture being worth ...
As the aluminum radiator gives up electrons it begins to corrode away ... these electrons are rushing to the copper block and the crud you see in the pics is the result of this. And while that material can be cleaned off and the block restored with no degradation. that crud is acting as an insulating material.
"Say it ain't so ... they must have away to stop this .." It's simple chemistry, it's science and the laws of science are inviolate. It happens in every water cooling system in cars, boats, power plants, PCs ... what differs is the rate and that is determined by:
a) The more similar and more "noble" the metals used, the lower the galvanic potential and the rate of corrosion
b) The maintenance schedule ... if you replace or augment the inhibitors as needed, then corrosion can be minimized
Now if the unit was installed just 4 months ago ... I doubt this is your problem. It was April when you made your 1st observations it's now the "dog days of summer". Were you keeping track of ambient temps ... even with the AC on here. it averages 4-5C hotter inside in August (74F) than it did in April (65F)
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