I was also facing these issues, after changing rings two times in just 1.5 years and spending over 30K, i decided to change the engine once for all, my engine was badly used
by my self, after a expensive hit I leaned few things, change engine oil on time, change oil filter and air filter, check water level and also check proper working of water body and tightness of belt, check radiator for leaks and radiator fan, also use engine on petrol before and after using CNG. and also install rpm meter and set your engine idle rpm. if i forgot any thing please add.