Great planes p51d .40 kit wabbles

[OpsOne]

I converted the great planes p51d .40
https://www.amainhobbies.com/great-planes-p51d-mustang-.40-size-kit
kit to an electric kit. It flies great with a 6s power plant. Lots of power and very ?spicy! But when it's flying at a decent clip it wobbles or yaws left and right when it is going straight. Any ideas on what may cause it? The only parts of the kit that got modified is that the motor compartment is a half inch shorter. And the wing tips shape is not a tear drop shape. Any thoughts

 

[Merv]

Any flutter in the rudder?

 

[OpsOne]

Any flutter in the rudder?

Not really

 

[Bricks]

You have flutter some place as you mention with speed the problem shows. Check all control surfaces for any linkage that may be loose, and hinges. You may have a servo that is having a problem holding center when put under pressure.

You mentioned not really when asked about flutter ( looseness ) from the rudder so what does not really mean? Does it or doesn't it?