Hey Sparrowhawk,
where is your controller connected to for power? some motherboard headers can be a bit more weak and be a limiting factor.
if you have one available I woudl recommend trying a 4 Pin PATA (Molex) to fan adapter to test the controller with full 12V being fed to it from the PSU to see if you see the same issue.
this would be the first step, if this does nto help then please reach out to our support team and they can assist you with proper assistance on diagnosis and look into replacement if necessary.