Cold water supply problem -- bathroom hand basin

The cold water supply to hand basin in our bathroom comes direct from the header storage tank in the loft above. This has been the case since the house was built in 1947.
Over the last 3 years or so the supply to the cold tap has tended to turn to a dribble, until I pump water through the pipe system direct from the tank under pressure using the garden hose.
This always solves the problem and water runs freely for the next six months or so after which the problem re-occurs.
My son replaced the tap and the cut off valve but still water did not flow until we pumped water through at pressure. Water is now flowing freely.
The water in the tank is perfectly clean and there was no sign of any debris in a filter we positioned to check water flowing from the tap.
There is no problem with supply to toilet cistern in upstairs bathroom which is also supplied by head pressure from tank in loft.
We wondered about air locks but how could that be?
Hope you can help.