Make sure you are using the proper servo save and that your linkage is not too tight.

You can always send the servos in and have us take a look at them. Sometimes a worn or dirty pot can cause this.

The 645 servos are very powerful but due to the 3-pole motor the centering is not as good as that of the coreless motor and or digital servos.

You might try the HS-525MG for touring.

Mike.