Hot water fine from boiler to all taps , but central heating not working , is fireing up for about 30sec then going out ,then firing up again after about 3-4min and so on , first rad hot , 2nd warm but cold else where , pump working can hear it and feel it pushing the water in the pipes in the hall coming from upstais. On advice have changed PCB and water flow sensor, but still the same .Any ideas All help and advice grateful