Small issue with DHW on Baxi 105e combi. Recently hot water has been hot to like warm. Seems flame heat is dropping on and off. HEX was cleaned 18 months ago and after reading through many forum posts seemed seemed likely cause. New HEX fitted and diaphragm service kit as was letting by. After all was changed still same issue, water heats to what I assume is temp and then flame cuts , temp drops and flame starts. Resistance across DHW thermistor at cold 11K and when hot at 70 degrees 5.5K. Diverter flow pipe for CH not getting hot so that side seems ok and CH is working fine. The issue happens on all taps showers etc so assume not related to shower valve . Could main HEX be an issue as system has been in many years and to my knowledge never had any inhibitor added . ? Any additional suggestions would be appreciated. Thx