You might want to check FMS settings for:
1. Default setting for cruise CofG;
2. Default setting for max thrust in the cruise. It could be 'Cruise', 'Climb', or 'Max Cont';
3. Buffet boundary 'g' limits (as suggested previously); and
4. Actual engine model and thrust rating.
I don't know much about the FMS 6000 but these parameters are relevant on other FMSs.
Good luck!