My boiler -Radiation Fuelsaver- switches off that's pilot light and everything when I switch the rads on.?

My boiler -Radiation Fuelsaver- switches off that's pilot light and everything when I switch the rads on. Hot water only is ok.My rads are switched on by opening a gate valve manually..All was working ok until i had the mains gas off so therefore the boiler was off too. The boiler fired up no probs and did the water as required but then I put on the rads and after a feww mins - off it went...Fires up ok but same again every time...Just ruined my pump grundfos 50/130 thinking it wasn't pumping so fitted a new 50/60. Just tried that and ok for hot water but when opened gate valve for rads it went off again. Could it be an air. The system is old but works usually and has a "TiTan" i think expansion mini tank thinghy.. Thanks KPM