I had this same exact problem on my Saito 65. It ended up being a tiny bit of slack in the throttle pushrod. I had drilled out the servo horn to accept my steel pushrod linkage and the hole was just a little to big. This combined with the engine vibration caused the throttle to be erattic at certain rpms. You could just put your finger on the linkage to stop the vibration and take up the slack and it would run fine. Also it would idle nice and smooth and transition just fine but when you decided to leave the throttle alone at mid to full throttle it would jump around. Almost sound like it was surging.

Hope this helps you

Jay