I sort agree Brian, I would not control a system as descibed, but what do I know.
With how it seems set up, and the temp settings, there simply is not enough run time to introduce energy energy into the underfloor (thermal storage), so regardless if the pump runs or cycles, water temp will fall quickly.
This system also has rads, so heat is lost very quickly from these (no thermal mass)
There seems to be no method of adjusting diff or dead band on these units.
All problems can fixed with $$$$
Normally, i control on return water temp (cold into the heat pump) and have room control via a stat in the concrete (Not in the air, no good for underfloor heating with a heat pump)
Increaseing the flow through the cond, would reduce the TD between water in/out and would introduce more energy into the floor, and make the system more stable, but at high flow noise seems to be a problem.