Timer issue or PCB issue?

Hi,

I have the analogue timer set up to control my hot water. Recently I've noticed that when it reaches the time to start up, it has trouble. I hear the PCB click to fire the boiler, and the boiler fires up, then it clicks off straight away and the boiler goes off. It will do the same thing in quick succession maybe 5-6 times before it stays "on". I have just had to have my boiler PCB changed last week and I am thinking maybe it was damaged by this constant on/off the boilermate was making it do.

This doesn't happen when I switch the rocker to "constant" hot water - fires up straight away no problem.

What do you think? PCB issue or timer issue? I'm leaning towards timer with the fact that it fires up no problem first time on "constant" mode. I think i'll leave it on this setting for now as I'm scared of damaging the boiler.