I've the same problem - have found turning the boiler down seems to reduce frequency of cut outs. Got mine set on 2 - only draw back is the radiators don't get so hot so it can take longer to heat house from cold.
The fact that it locks out only after it has been off for a while points to machanical faulty
the most common fault for lock out is dirty fan drum and slightly blocked venture' causing failure of air pressure switch could also be aps itself
Halstead parts are expensive,not advisable to replace willy nilly
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.