Cant say for certain without seeing it but I would say there could be a problem with your boiler thermostat.
It seems as though your boiler is getting to temperature and not turning off then activating the overheat safety device. This too could be over-sensitive and need replacing.
Try turning the boiler stat down a bit. If that doesn't work you need a corgi repair engineer to check it out.
Slugbabydotcom
November 2007