My new boiler doesn't work properley?

My new boiler seems to be working some of the time but not all the time, i have set the times for on and off but some days it will come on and others it won't, the red light is sometimes on or flashing, it will work if i put it on manually but that does't help as i need the house warm first thing and when i get home from work, it doesn't work on a tuesday/wed and thursday morning but then works fine until sunday when it stops again, i have double checked and even re-set the clock but nothing seems to work, the engineer says he will fit another clock as it has only been in a few months, will this sort it or do you think ther may be another problem?