I have a Hayward HDN400 pool heater and lately when heating my spa it shuts down around 90F and then comes back on 2-3 mins later. It’ll keep doing this every few mins until it reaches set temperature. I have installed new filters and checked everything else that could restrict flow which is all fine. Pressure in filter canister is also in regular range.
We’d be glad to assist you. Are you seeing any error codes when it shuts down?