The KSEA to KSFF is a flight plan named dpl.PLN. Do not delete it. The 707 will always load the dpl.PLN when it is loaded it into FSX. Without that plan, the 707's Doppler Navigation will not work!

The thing to remember is this. The 707 gets set back to a Cold-n-Dark setting every time you go to some areas of the menu. When it does, it automatically loads the dpl.PLN!
So my advice is to make the flight plan the last thing you do before commencing your flight. Or at the least, as you are leaving the Gate, make sure YOUR flight plan is the plan in use, and not the dpl.PLN!

Mark