Rotation Question / Error?

Rotation Question
I am experiencing some strange behavor rotating objects in Torque. I have
created my own class called Aircraft that inherits ShapeBase. I have written my
own processTick member function to allow me complete control over the path
of my object instances. Of course, the packUpdat and unpackUpdate member
functions are specific to the class as well.

I now need to be able to rotate the aircraft in X, Y and Z. I have removed the
Z only restrictions normally seen on rotation seen in other classes derived
from ShapeBase.

I can now rotate my Aircraft objects easily but have noticed that the
rotations around the X and Y axis do not behave as one would expect.

Rotations around the Z axis work fine. Specifically, the rotation occurs
around the center of the object.

Rotations around the X or Y axis do not occur around the objects
center. Instead, the rotation occurs around a point a significant distance
from the object center.

Here is a very simplified Aircraft::processTick() function:

void Aircraft::processTick(const Move* move)
{
mCurrentStep++;

//Misc. setup
setMaskBits(PositionMask);

F32 dt = TickSec; // Amount of time between ticks

// Get current position
Point3F pos;
mObjToWorld.getColumn(3,&pos);

// Calculate desired Velocity
VectorF vel;
vel.x = -15.0f; // move across the sky along the X axis
vel.y = 0.0f; // maintain constant Y axis orientation
vel.z = 0.0f; // maintain constant altitude
mVelocity = vel;

// Calcuate new position
pos = pos + mVelocity * dt; // maintain constant velocity

mRotX = (mRotX > 6.28) ? 0 : mRotX + 0.5; // rotate around X axis
mRotY = (mRotY > 6.28) ? 0 : mRotY + 0.0;
mRotZ = (mRotZ > 6.28) ? 0 : mRotZ + 0.0;

// Update transform
MatrixF mat = mObjToWorld;
mat.set(EulerF(mRotX,mRotY,mRotZ));
mat.setColumn(3,pos);
Parent::setTransform(mat);

}

When I modify the mRotZ value it works it should. When I modify the
mRotX or mRotY values the error in rotation occurs.

So, anybody have enough core engine experience to point me in the right
direction on this one?