Ravenheat RSF 100ET

CH operates without problem

DHW developed the following problem earlier tonight

DHW short cycles on & off in roughly 30 second bursts. This can be 'cured'
by backing off the DHW control on the front panel to about 40% of its full
value (which is what it is usually set at) - which reduces the temp. at the
taps to tepid. The main overheat stat (which is resetable) has not tripped.
The mains water pressure is unchanged so I have initially discounted low
pressure problems. We live in a soft water area so no scaling problems.

Could this be a thermistor problem ?

Neil