There can be a thermostat on the heater. Remove it or change the ring.
The cooling water hose is reversed. Check against the sketch in this book.
Too much mash in the boiling vessel. Allow for at least 200 mm space.
One litre of boiling stones (i.e., Rachi rings) in the bottom of the boiling vessel spreads the heat more evenly and reduces surge boiling. Add some anti foaming agent for example M10 Stabil. If you have a water well and pump, water pressure can vary which can cause surge boiling. There are several solutions to this.
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