No responses? Ill have a go.
I dont really know but I read some things in the past that might be related to your problem. From reading parts of the following two URLs, and from what you said, to me it sounds like the boiler is slowly losing water and it is not being replaced. When the boiler water level drops enough, the thermosyphon stops working(this is my understanding on how it works - could be wrong). When you "flush it for a second or two" the boiler fills up enough for the thermosyphon it start working again. The drop in water level could be from some slow leak in one of the valves - or maybe it is just a normal drop in water level and the problem is with the boiler refill not functioning adequately.
Note that the above are from different machines to yours but the operating principals should be similar.
So what to do about this? Probably just call your supplier and ask them for advice on what to test and what they recommend you do. I am just guessing and could have this all wrong. A supplier/repairer will probably be able to diagnose this very easily. On another thread I think you mentioned that you got the machine from Chris a short while ago so I am sure he would be happy to help you. Then tell us what you did to fix it.
Other than that, maybe someone else can suggest what to test and look for. If you want to try searching for more info on this problem, also try this spelling: thermosyphon