This is something that has been seen on some heatpumps…
- Samsung HT Quiet R32 8KW / EmonCMS - COP vs simulated COP and zigzag pattern
- Daikin 8kw flow T and power are hunting and will not settle down
…though those typically see a matching oscillation of the electrical input at the same time. That makes me wonder if the fluctuations are real, or some noise on the flow temperature sensor.
What model of heat pump have you got, and how is performance being measured?