Problem with my Potterton Profile 60e?

My 60e tries to come on and it sounds like the burners are about to fire up but they don't. the boiler switches for for 2 seconds and tries again. I have changed the air pressure swtich and probelm still persists.

The thing is, when I gently knock the boiler electronic centre (that is the circuit board - big black bos on top of circuit board) at the time its about ot fire, it works.

However, when it reaches the thermostat temp it cuts out and then goes through same routine until I bang it again. Any ideas why this is happening. thanks in advance for your help.