I think "the shudders" you're seeing is the one caused by play of the main rotors.
The two blades attach after sliding over the pivot pins on the rotor head.
If there is excessive play side to side (try sliding it side to side), you get an action like that.
One of mine did it.
Solution, add a washer of some type to one (or both) of the pins that the rotors slide over.
Make sure it's not think enough that it causes the blades to bind.
They need to pivot completely freely.