Hi.
I've had similar issues with our engine (MB), especially when I tried to use it at rich setting, 4-stroking all the time.
My problems were caused by the engine running too cold. When engine is far out from thermal balance, it will heat up during the first maneuvres. Heating causes better burning which causes higher RPM.
This happens to me usually in the morning when temperature and relative humidity is low but there is still water mist in the air. Our engine is very sensitive to that, mainly because of integrated AAC construction & piston scavenging, which makes it propably the coolest engine on planet. But combined with ultra light weight, it's not an easy combination.
Anyway, my solution for the leaning-out problem was to increase compression. Also, as I have lots of power to waste, I also build up the stability by choking the exhaust end.
I have fixed quite a few Discovery Retro engines for people suffering from same kind of problems. The most common repair is to set back the factory settings (venturi, compression) and to remind of how important it is to use correct type of fuel. For example, several synthetic oils don't work with DR at all, causing erratic running. Then people think that the engine can be tamed down by adding some shims. That messes up the thermal balance and lowers power. The next natural step is that they drill the venturi bigger which messes up the harmony even more.
So, providing that your fuel system is OK, I recommend to increase compression step-by-step, and perhaps to try with all-castor fuel.
Lauri