Hi, yours I think will be the last of the Carver Cascade 2's, these have two 'button' thermostats which control the normal temperature (70C) automatically, and an over temp (90C) with the manual reset button I assume you are pressing.
What I think is happening is the normal one is not working and the over temp is therefore doing it's job, if I am right you should find the water will be very hot when it trips?, much hot than when on gas.
A new 70C stat is
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