vaillant 221H hot water problem?

When running bath water the water starts out hot and then goes cold and then hot again. When watching the main burner it comes on full and then a few minutes later cycles to minimum (not pilot) and then obviously cools and goes to full fire again only to reduce to minimum after a few minutes on full fire. My thinking is that there is an overheat sensor on the boiler hot water output that is indicating(incorrectly, that there is an overheat problem - can any one suggest what might be gouing on so that we can get this fixed.