Well, I removed the spark plug and yes, it was oily and probably fouled. Put a new one in, but sadly, the problem remains.

Thanks for that Rich. Had a check and the gap seems ok.

Well, I'm about out of ideas. Pretty certain that the problem is with the EI since the engine appears to be mechanically sound and I've cleaned out the carb (twice). The engine was running fine before, but started showing problems within about 10 tanks. Guess I'll have to swap out the EI and see if things change.