Sharkbait123

CPU header. If the pump fails, the AiO is useless. The CPU header is designed to recognize when fans and pumps fail, and kick on safeguards to prevent extremely overheating the CPU.

Sometime between today and some time later. 

Good idea, i'll definitely try that.
 
My case is an InWin GT1. Not the best, but it does the job to a decent standard.
 
Thanks for your help! 

So you have a negative air pressure setup going? 
 
You can try stressing your 280x with the side panel off. If temps go down, then its an airflow problem. If its still too hot, then something is funky of your card's heatsink. 

if the cable only has 4 wires inside the RJ45 plug, it cannot exceed 100mbit.
 
But check that you are using at least cat5e with all 8 wires  
There might be a broken wire somewhere in the cable, so test with another cable too  

I also have a R9 280X, and in full load it gets around 76 degrees Celsius. My case config is 1 fan intake in the lower front of the case and 2 fans exhaust in the top back.