2 channel heating programmer resets when switching load on

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Called out to look at a heating programmer (Honeywell 88608) that is reseting back to factory settings. Controls the hot water tank and a couple of towel radiators. Area has frequent power cuts and thought it could be a dead backup battery but replacing it didn't fix the issue. Replaced the 2 channel programmer with a different model (Sangamo CHPPR2) and noticed when setting it up that it reset back to factory settings when switching the heating load on. Checked all the wiring which is visible as there is no thermostat for the towel radiators and no issues. The hot water is controlled fine with the heating wire disconnected. Replaced the two port valve head thinking there could be an issue but still the same. Any suggestions? It's an S Plan system with 3 heating zones and hot water, I put the towel radiator zone valve on with another heating zone that has a Honeywell programmable thermostat (T3H110A) and that switches the two zones fine together. The T3H110A is battery powered for it's controls so thinking that is providing separation from some electrical interference that is causing the other programmers to reset?