air trapped in worcester bosch 40 cdi ?

My worcester Bosch 40 CDI goes down nearly every time I manually switch it to radiator mode. It may run fine for an hour or two but eventualy it switches itself off with the normal array of flashing engineer symbols and f7. When it goes funny it makes a very loud repeated whoopping type sound as if there is a massive air block inside and the repeated noise makes you swich the whole system down as it doesn't stop until you do so. this has gone on for a year or so. My central heating has new radiators but have the microbore pipe
connecting them. I have bled all radiators but there is no apparant air in the system. The boiler runs fine when just on the hot water mode for the taps. The boiler is serviced every year to remove the aluminium crud from the filters. I feel like ripping the boiler out and putting in a baxi boiler in. I'm that fed up. Can anyone suggest what the problem may be