Ravenheat RSF100 DHW problem

CH operates without problem
The DHW developed the following problem earlier this evening
DHW cycles on and off in roughly 30 second bursts. This can be 'cured' if the DHW control on the front panel is backed off to about 40% of its full value (which is where it usually is) - which reduces the temp at the taps to tepid. At first I thought this was an overheat problem but the overheat stat (which is resetable) has not tripped. The mains water pressure has not changed so I have initially discounted low pressure problems. We live in a soft water area so no scaling problems.
Given that the main overheat stat doesn't pop are the likely culprits either the DHW sensor (thermistor) or control (potentiometer) ?
Neil
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