Hi Bruce,
*DISCLAIMER: I'M NOT A COMPUTER GUY*
I had a lot of issues before with FSX on Win 8, but found a rather obvious workaround that has let me use FS Passengers, FSX, and PMDG aircraft at the same time (a fatal concoction in the past). I reset my PC completely (that means reinstalling Win 8 from scratch and removing all my files). I had an unrelated computer issue that was just easier to solve this way.
When it came time to install FSX and all my addons, I ensured that I ran all the installers in Administrator mode and compatibility mode set for Windows 7.
Now they all jive like a charm!