Another Worcester 24i problem?

i have the above boiler which just before xmas i had a new pcb fitted, everything was fine until now. all of a sudden after the central heating or hot water being on for anything from 10-60mins the boiler will just stop, the flames stop and it wont reignite for ages, as if until cooled down? everything seems to be working, ie pump, fan etc thefront does seem to get hot to the touch but i'm unsure if this is unusual as i never checked before? any help appreciated. if you need anymore info i will provide. thanks