Potterton Profile 50e struggles to light?

Hi,

Our Potterton Profile 50e Boiler is showing the following problem:

When the system switches on (heating or water) the boiler always starts fine the first time. There is a first clicking sound and a fan starts running and after a few seconds there is another clicking sounds and the boiler lights.
When it reaches it's temperature it stops as expected but the next time it tries to start the following happens.
The first click can be heard and the fan starts running for a few seconds. However the second click does not come and the fan stops running. After about 30 seconds this procedure repeats itself over and over again.
The weird thing is that if I switch the system off (either by turning the white knob on the boiler to 0 or the main heating controls), wait a minute or so and then switch it back on the boiler fires up fine again but the next ignition sequence fails again.
Sporadically even without changing anything the boiler start up by itself but this is very unreliable.

We've had the PCB replaced about two years back. Does anybody have an idea what's causing this behaviour? Any help would be much appreciated.