vokera 29 HE hot/cold water?

when running hot tap fully open, burner shuts down and water runs cold for a moment then burner fires up and i get hot water again, it does this repeatably.
if you close down hot water tap by half (reducing flow) burner will remain lit and hot water fine. burner only shuts down when hot tap fully open.
central heating not affected and running ok

firestarter
October 2011