Mark,
" Are you loading your FSBuild flight plan via the FS Flight Planner?"

Yes.

I just made a final test flight KPVD-KILM  FSflight plan loaded and A/P Nav Mode engaged. 17 way-points Flight completed with perfect navigation through A/P NAV mode.

There is no NAV/GPS switch in 707 and not needed.
I am sure there is a hidden GPS system in 707 Navigation. Nothing wrong to have it, but INS/Doppler will loose priority for navigational preferences.

Sanal