My auto pilot works just fine so I would have to say that your problem is probably with the other add-ons and not with Captain Sim's 757 Block F.
I would suggest that you make sure you have the latest FSUIPC, uninstall your add-ons and Cpt Sim's 757 BF, re-install Cpt Sim's 757 BF,  start the sim with the default Cessna at KSEA, fly the Block F to see if you still have the problem, then re-install each add-on until you find the culprit.  (Also remember if you want to start with the 757 "Cold and Dark",while you have the Cessna loaded, turn off the master switches and ignition, but don't turn off the avionics. That may cause problems too)
I don't use FS Passengers, but I understand that conflicts with airliner sims are not unusual with it.
I've used Captain Sim's support before with success.  I know that software support can become a massive headache if there is not some system of formatting it in place.  I've seen it on forums of other simulation developers and could actually hear the pain and frustration in their voice from dealing with massive amounts of customers on the same issue when I talked to them on the phone (actually referring to the support of the one you claim to be "going back" to).  They have a very good 767 model and there is nothing wrong with that approach to supporting it, if that's the way that they want to handle their support, but there is also nothing wrong with going by Captain Sim's requirements in acquiring his support when it is needed.  
There is no way that support can pinpoint a problem it they don't know the exact environment that their product is in.  It may seem like a little more trouble, but I can see the need for it.