Grundfos negative head pump pressure issues

You say its sensitive to HW temperature change, normally a sign of cavitation, can you run it, both hot& cold sides via separate non mixer taps until the HW cylinder starts running likewarm/cold then compare both flowrates.

Have you checked pump strainer(s), also, if possible, post a schematic of how the hot supply is supplied, from the HW cylinder take off right to the pump.

This is the recommended method of installing it but check your inlet strainer anyway, also check that the HW cylinder vent isn't dipping down into the water in the cold water storage cistern.

Yes, but the issue that can arise with high performance pumps is that air is sucked by the pump through the vent which rises vertically from the cylinder and hangs over into the cold water storage tank. This would cause the symptoms you describe.