PMDG 747 in Vista or XP 64-bit

Hello, I have much problems with my 747 of PMDG in 64-bit Windows. Doesn't matter if it is XP 64-bit or Vista Ultimate 64-bit. The plane doesn't react to what I ask it to do. When I descent from 24000 ft to 7000 ft and I want to fly 250 knots, the plane does descent but not the right way. It doesn't stop descending after 7000 ft. When I don't pull up myself the plane crashes. The speed is not correctly too. I know how to fly the plane because I did it on XP 32-bit a lot, and I never had a problem. Is the aircraft not working correctly o a 64-bit system? When I search on forums I see people who fly the 747 on a 64-bit system without any problem. I have the newest version. Also the 747-400X Upgrade for the FS2004 747 is installed but it doen't matter. With or without updates I keep having this problem. Can somebody help me? I want to fly the very nice 747 again without having problems. Thanks a lot! Kind regards, Ronald Rol