In all honesty, I can't see why it would only happen to the 767-300 RR. In the aircraft.cfg file, the only difference between the 767-300 RR and the 767-300 GE or PW is the sound file (sound.rr) and the exterior model (model.rr5).

[fltsim.5] <--This is the 6th plane listed in the aircraft.cfg. all the rest are GE or PW
title=CS 767-300 Captain Paint5
sim=B767-300RR
model=rr5 <-- exterior model only. VC + Pax Cabin is the same for all of them (interior model is directed to model.ge5\cs763ge5_interior.mdl)
panel=
sound=RR <-- the sound.RR is different to the PW and GE (sound.GE)
texture=british
kb_checklists=
kb_reference=
atc_id=G-BNWW
atc_airline=speed bird
atc_flight_number=763
atc_heavy=1
ui_manufacturer="Boeing"
ui_type="767-300 RR" <--This just displays it in the aircraft list as a 767-300 RR so it does nothing really
ui_variation="British Airways"
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
description="767-300 Base Pack v1.4\n(c) 2009 Captain Sim www.captainsim.com"
ui_wglts=1

The highlighted lines are the only real differences with the GE and PW.
You say that you only fly the BWA 767-300 RR. But have you tried the same situation with PW and GE models to see if they do this too? It might be all your 767-300's.

Mark