Problem with Britony combi se 80 low water pressure CH?

Pressure switch tested ok as it makes a circuit the P.C.B has been changed with a brand new board but the same fault keeps on reoccurring. Have simulated another problem and that seemed to have cleared the original low pressure
problem and the boiler worked for two days but the low pressure problem as come back again! what could cause
this type of problem?

Andy H
March 2009