Hotpoint Ultima CDT80 - cutting out?

A few months ago my Ultima CDT80 was displaying f12, I followed the reset advise and it has worked great up till now. My new problem is that no warning is shown on the display, my machine still heats but will mow only work for approx 30sec and then cuts out. It restarts when you press the start button but only for 30sec. Has anyone any suggestions as to the cause of this new problem?