Symptoms are rather like those I describe in a problem I had with a Halstead boiler in answering someone else's query. Text is reproduced below. Please adjust for your context.
"I had this problem with a Halstead a while ago. This boiler had a low-capacity heat exchanger (this is design, not a fault) that required a good flow of water through it. Pumping the water round the whole central heating system was too slow for it; the water in the heat exchanger reached close to boiling and then tripped out necessitating manual intervention to restart.
Basically, this boiler would work OK if enough water moved through the heat exchanger so it could heat it up and the pump could move it on before it got too hot. When the incoming water (return) was at a high enough temperature, the heat exchanger would turn off, the pump would still circulate the water and when the return temperature was low enough, the heat exchanger would start up again.
All this has to happen at a temperature below that when the safety over-heat cut-out operates. If the water flow is too low, the safety cut-out works quicker than the heat-exchanger turn-off (and so it should!) and the boiler shuts down.
Solution (from the Halstead documentation) was to insert a bypass connecting the boiler flow to the boiler return. There was a control cock in the bypass allowing adjustment to the flow that was allowed to bypass. I placed the bypass next to the motorised valve (between boiler and valve) which was adjacent to the hot water tank and in pipe length, about 3 meters from the boiler, which happened to have the pump next to it. Radiators started to connect to the flow/return after this 3 metre point.
Result was that the pump could move enough water through the heat exchanger to allow it to heat up the water and move it on before the overheat tripped. Not all of the water took the bypass route so the rads still heated up in the normal way - they might have taken 5 minutes longer.
It would be good practice also to ensure that the control system is configured to make the pump over-run (i.e. continue running for a minute or two after the heat exchanger decides it wants to stop heating for a while).
Possible alternatives: fit a new, more powerful pump that keeps enough water circulating and avoids the problem; do a drain/flush - how much sludge do you have in the system that might be slowing down the flow."
Hope this helps.......
Peter
