Glow.worm 30SXi F13?

4 year old boiler started to trip out with a F13 fault on display. British Gas have replaced main PCB and wiring harness, next day fault F13 back again and the system has tripped out. This only seems to hapen in the evening when the thermostat is turned down, timber switches on system and when we turn up thermostat later no heating available and F13 fault back. Please help