Guys, this is weird it took me quite some time to figure it out : after extensive testing I found out that as soon as I climb out to FL350 (at OPTIMUM not MAX from FMS) the AP loose track completely, cannot even change track 10° at a waypoint without going in a phugoid left and right from track with increasing lag, even switching off LNAV and trying to steer with HDG select gives no result, plane do not respond at all. When I descent back to FL340 plane steer to catch the track immediately anf follow it perfectly. Was tried on 2 different FLP the triggering factor was always FL350 and same applies for CS 757 for instance. Anyone experienced this already ?