UFH blending valve reduces flow

[oz-plumber]

Could it be the Pump.
Isn't there an irregular occurrence of the pump spindle and the impeller parting ways.
Or the impeller not spinning at the same revolutions of the spindle.

The only way to check is pull the pump body from off the pump head

 

[John.g]

The mixed temp is fine at 40C so needs the manifold pump to be OK for this?