mountfield 1740 rideon running fault?

Hi
Have a situation on my mountfield ride on mower 1740 hydro,it starts fine but after approx. 5 secs a buzzer goes off and the engine cuts!! it then immediately starts again but does the same until suddenly it will run ok and keep running, has anyone any ideas please ,engine is a 17hp single briggs. same machine as castel garden tc102
thanks