I agree, it might not be a bad idea to adjust the low end and really take a good look at the fuel system, including inside the tank.

Also, your picture shows no cowl but are you flying it with one on it? if so, you may need better air flow for cooling. i have made the mistake of not cutting a big enough outlet only to have an engine overheat and quit during flight.

You may want to throw in a new glow plug to just to rule it out and don't forget to check all the engine screws. I've seen cylinder heads come loose and make engines run funny.<br type="_moz" />