I have switched to P3D since in P3D I don't have this problem anymore but i still have the PMDG777-200/300 and I would like to fly it so I try everything.
I have FSUIPC but FSUIPC uses the same things as FSX to connect to the operating system so i don't see why it should help.
What I have found is that always that I alt-tab without the sim being paused, it automatically loses connection and I haven't been able to have it back. However, I have been trying to pause it THEN do alt-tab, return again and unpause and it keeps it connected! I have tested this not extensively, tomorrow I will try again to see if it is consistent or not. I am running FSX as administrator, with Windows 7 compatibility in Win 8 Pro 64 bits.