I don't see how it can work sometimes and sometimes not.  I'm using Vista64 and have FSUIPC v4.50 which works fine.
Pete Dawson says that there are some interum versions also available, but 4.52 will be released in a couple of weeks and he will no longer be supporting 4.50, so if there is a problem with the FSX Captain Sim 757, and newer versions of FSUIPC, they need to get fixed within the next two weeks or it's going to get left high and dry.
He says that some others are complaining about problems with the autopilot and new versions of FSUIPC and that Captain Sim needs to get a hold of him so they can fix the problem.