voll, do the same test, only make sure that you use auto level.
Hity the X key and dont move the stick. You will fly straight and level. This will get rid of any tendency you have to move a flight control. IE every time you move a control (rudder, aileron, elevator) it comes at a cost to you in Energy loss. Is it possible that you are kind of twitching through the air, thereby losing a little energy?
Ghosth is bang on with the calibration recommendation. If you are getting any spikes at all you will have a corresponding movement in the associated control surface which = E loss.
Also, pay particular attention to what your manifold pressure, and RPM are doing. Do you both have the same indications?
Does your airspeed indicator read the same as his? etc.
Hope some of this helps figure out whats going on for you.
Cheers,
RTR