FMC and A/T Question, Please Help

Hello,I've had PIC for well over a year and enjoy it very much. I've noticed that when I load a flight plan into the FMC from FSnav, it sometimes changes the altitudes from what the flight information in FSnav gives. I consider this probably normal, but sometimes it changes it quite a bit.For instance, on a short 200 mile flight, it changed the altitude from a requested one, by me, from 20000 ft. to 40000 ft., quite a change for a 200 mile flight. I normally go through and change it back, but strange things will usually happen on a flight if I do. The plane will either catch the G/S but won't descend, even when switching the AP off, or like last night, the A/T got stuck.The A/T last night was strange. While cruising, the plane kept building speed, even when I tried to intervene. I disconnected the AP, FD, and A/T. I manually moved the throttle from up and I could hear what sounded like a second set of throttles kicking in.I've had good luck importing flight plans into the FMC and find this recent behavior strange. PIC has always been very stable for me. Can anyone out there provide me with some insight as to why this may be happening?Sincerely,Hans [email protected]