Potterton Suprima 30 cutting out- green flashing light?

Hi, I hope you can help. I've just had a new PCB fitted and now when the demand is on for either the hot water or heating (or both) the boiler will fire up and get warm, it usually takes about 30 minutes, and then it will cut out- the green light will flash constantly-before the red light comes on, the boiler will temporarily knock off, and then reignite if the demand is on. The boiler will then work for approx 4 minutes, before the green light starts to flash again, then the red light will come on, then it will reignite and come on again for 4 minutes and so on.

I have also noticed that I can hear fast flowing water in/around the boiler and before all the problems with the PCB I never heard water before, or had the cut out problem.

I've had an engineer out to investigate who says there's nothing wrong and its normal to go on and off so frequently, but I'm not convinced?