Vaillant turbo max going into lockout??

Iv got a customer with a vaillant turbo max,black front green control buttons,gc 4704425.wen heating is on,they run hot tap but sometimes goes into lockout,it happens more wen heating has been on for longer periods.iv changed NTC,differential switch,divertor valve opening as it should,Iv even turned the pump over-run down on pcb???.oh changed pump as well any ideas?????