I would try this:

1. UNDO the elevator push-rod from the servo horn.

2. mount a new horn on a small piece of balsa or ply using a
wood screw to where it rotates as it normally would hooked
to the servo.

3. Hook your pushrod to this assembly and tack glue it in place
to where it duplicates being connected to the servo.

4. Center the elevator and run the engine. If its heat expansion
the surface should move as stated.
If it doesn't then it could be the servo.

Just a thought.

W.G. Hunter