sounds like its jumping cause its a tad too rich on the low needle,you may wanna try leaning it in very small increments till the twitch goes away.its normal for it to have an ocassional little burp but not an oscillation thats in time w the engine coughing.
Using a non resistor plug can cause this too,ngk bmr7a plugs are optimal for gas helis,bosch,champ,autolite will cause the same condition w the tail,even worse on a ergo gasser.

The 550 works ok,just dont use a 8417 w it........the jumpiness should go away when the machine runs smooth.
Always do carb adustments after the machine has been warmed up or has been running for 3 minutes.The mixture tends to lean out a little after the engine gets warm.
It usually takes a gallon of fuel to get the engine almost past its break in point......
I put some basic gasser info on my website that you may want to read,its basic info,and may answer a few questions and give you a few pointers.like most sites its still under construction <g>.
Hope this gives you some of the answers you been looking for!
M