OK after reading the autopilot *only* thread I removed all FSUIPC (V.493) in the module folder of FSX, repaired the 767-300 base installation and tried again : seems the problem disappeared as I can get to FL360 and take 30° track change without any issue with LNAV or HDG SELECT. It seems it was linked to FSUIPC somehow. Now I must find a friendly FSUIPC for Flight Sim Commander to work on my second screen - I always set FSX in windowed mode only because of FSC.