The problem I have is that the temperature of the water goes from
either freezing cold to scalding hot in a cycle of probably 30-50
seconds. No amount of adjustment of the control helps me. any ideas
anyone?

Probably the thermostat module needs an overhaul.
Even if the hot or cold stop the 415 valve will not change the temp by much.
Unless its very old (20 years) the threads on the valve module are LH.
You could undo it and descale it and see if that fixes it.
Otherwise send it away to be fixed.