vailant turbomax 242 overheating?

I have a 10 year old Vailant Turbomax 242 boiler which works fine in all respects except that if I leave the hot tap on for more than a minute or two, when I then turn the tap off, it will not fire the boiler up if turned straight back on again. After five minutes, it then works OK again. It's behaving as though it's over heating and not firing again until it cools down. The pressure gauge is always at 1 BAR, and the temperature gauge only goes up to about 60. It has only been doing this for about a month. Any advice would be greatly appreciated.