protherm 100 has had 4 parts relaced now worse?

boil has had parts replaced trying 2 stop f1 but now heating ok still goes on f1 but r water only stay hot 4 couple of mins then cold then hot again we can not have a bath cause more cold then hot the part replaced r overheat thermastat and 2 thermistors an some other part ! can anyone help please r engineer is going 2 have nervess breakdown thanks colette