Potterton netaheat 60e sometimes wont light up when warm?

my Potterton netaheat 60e boiler as after about an hour starts to not fire up straight away , sometimes will take up to 30 secs
more recently on 2 ocassions has given up trying untill cooled down and then will restart . it makes all the right noises although fan is a bit noisy , and an engineer has changed the pressure swith and this has not altered the running habits at all . Please post your comments all donations greatfully accepted