MCP Help

Forgive me if this is in the wrong forum but this question does relate to many FSX aircraft. I'm not an expert at flying any of the heavies, although I do know enough to load a flight plan in the FMC/CDU, program the legs, SIDS/STARS and autoland , for example the LevelD 767. One thing that I really don't have a firm grasp on is the MCP. Some of my addons, the MCP functions work flawlessly. On others (say posky 737, Feelthere 737 and other complex addons) the aircraft(s) don't seem to obey the ALT HOLD or LVL Change correctly. They either drift altitude quite a bit, or don't honor the LVL Change selection at all. Does anyone have a good resource or link to aid me in finding what part of the flight planning I am doing incorrectly? Thanks.