Efel oil stove flames die down for no apparent reason?

We have moved to rented accommodation which has an EFEL Harmony oil stove. I light the stove, and it seems to work fine for maybe an hour with effect coal and logs glowing red, but then for no apparent reason the flames die down to nothing even though the boiler may be on the highest setting. Normally at this point I just turn it off fed up, but when I come to light it the following day all is ok for an hour or so and then the same problem occurs. Why should this be and how I can resolve this. The stove has a back boiler with flow and return pipes, on one of the pipes is a thermostat, this only seems to control the central heating pump.

Please help, there does not seem to be any useful information on the net.....