Panel Temp Display

Help with my ISAR commissions that 35 boiler, hot water is very inconsistent?
I have an Ideal Isar HE35 boiler Combi. When running a bath or shower the temperature goes from hot to cold and again every 10 seconds or less. The pressure is not affected. There seems to be enough hot water despite little things, like washing hands or dishes. When running a bath or shower to light the burner is then shut down and then I imagine that is when the temperature is warm – cold – warm, but do not know why. When not used or displayed''when in use shows 'd'. Can someone hep me what could be wrong? I called my rental agency, but say that what looks like a timer problem and will be charged if your fault? Harsh. I tried nabual the user to download, however, the control panel in the illustrated manual is different from mine. I have no facility to control my water temperature. Please help. The boiler is only two years, although that has not been answered yet
Probably a thermostat crazy, maybe I should think of service as well.
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