Central Heating Boiler stopping - Glowworm Micron 60FF

When my central heating is put on for more than a short time (one or two hours) the boiler invariably stops heating. Pressing reset on the boiler forces it back into life.
The strange thing is that if I press 'boost' on the controller the central heating it will go on for a full 60 minutes. After that if I press boost again it'll go on perfectly for another hour, and so on throughout the day.
So this suggests to me that the boiler is probably okay. But as pressing the boiler's reset button sorts it out, this suggests that it is a boiler problem.
Any ideas anyone?
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