hot water not getting hot enough, please help?

the DHW has a problem in that, it does not get up to correct temp..the burner keeps cutting out when the temp of the water starts to get warm.

replaced thermistor, and checked the diaphram in diverter..pins of diverter are moving correctly and operating the primary and DHW microswitches..its just that the burner is not staying on long enough to give hot water...