Worcester 28i rsf intermittent fault?

My boiler has an intermittent fault. It fails to fire up every so often. When this occurs, taking off the low pressure pipe from the air pressure switch resets the switch to neutral, the fan starts running and the boiler fires up. Replaced the pressure switch a year ago and all worked fine for a while but now same problem occurs. Don't think valve has failed since after reset, all seems fine for another couple of days. Don't understand how vacuum can remain without fan running.