912 run rough on climb or heavy load

I have a 912ULS in a RV-12. It has an electric fuel pump that runs 100% along with the engine pump. I have experienced the exact thing in hot weather when using winter blend fuel. The cause is vapor lock. I can force it to happen by turning off the electric pump. Bringing the rpm back to 5000 will usually get the engine running smooth. I can maintain altitude at 4350 so can still climb at 5000. It doesn’t happen often, when it does I am usually expecting it due to the conditions, fuel / temperature. 

The things that add to the possibility of vapor lock. High fuel temperature, hot cowl area, lower fuel pressure, high altitude, winter blend fuel with a lower boiling temperature. 
Things that reduce the possibility of vapor lock, summer blend fuel with a higher boiling temperature, lower altitude, lower cowl temperature, (don’t sit idling or with a tail wind), have a fuel system with a return orifice, higher fuel pressure ( higher pressure increases the temperature fuel boils). 

In my Australian experience with fuel vaporisation, it only happens on the ground, high ambient temperature (35C+)  after flight and short time between shutdown & restart (hot engine, has boiled fuel in under cowl fuel lines).

Hot start will require boost pump on, for extender period, before attempting start. Initial indication will be low pressure, often followed by pressure fluctuations/pulsing. Allow system time to purge vapour through return line. Do not attempt to start engine before pressure remains constant.

Do not increase throttle while attempting to start.

Extend run up time and be prepared for loss of power, during take-off role, when throttle wide open.

Once the vapour has cleared the engine should run normally - changes in rpm will have no effect.

I don't think that most of Australia has winter fuel.😈

Dave's engine symptoms sound more like an air leak in the fuel/carburettor/inlet manifold😈