This is very common on Onan 1800RPM single cylinder generators, such as 2.5LK and 3.0DJA.small variations in rotational speed caused by the non-linear rotational torque produced by reciprocating engines. This is most common with engines with a small number of cylinders since more cylinders means a more linear torque output.
Ray on my JB 7.5 on propane the spark plugs NGK B-6L i have them set at .025" at .018" it had a miss at no load once i opened the plug gap there is no miss at load or no load hope this helps.I switched to the NGK recently. The come out of the box at .030. Started set up and at no load no miss. At half load, no miss. (Approx 1 hour run). Removed load and started to miss but not as frequently as with previous Autolite plugs. Believe Brae set his gap to .028. Gonna try that next time I run set.
7.5 JB propane
Another thing I've noticed that sometimes causes flickering, poorly designed, triac "power controllers"The little flicker you see on generator power is usually one of four things: