>>What year & engine?
> 2002 Ford Taurus Sedan SEL. DOHC 24 Valve V6 Engine. AX4N Transaxle.
OK... the Duratec/AX4N shouldn't really do a lot of "hunting" at
highway speeds, regardless of topography. Even if it was, a
full-throttle application to effect a kick-down shouldn't be necessary.
Do you drive with the speed control engaged? Try that. If you feel you
need to help the drivetrain "anticipate" a big hill, do a quick, maybe
1/2 tap (TIP-in) on the accelerator pedal while acsending. The
transmission should shift out of "lock-up" in 4th gear. Let the speed
control demand more throttle as needed. It may then shift to 3rd gear
as needed, then back to 4th, and then lock up the converter again as the
set speed is reached and the load is diminished(lock-up is modulated, so
it may not be as obvious as on older cars you may have driven) which is
the "kind of -fifth gear-" you were describing.
See how it all works together by letting the speed control do all of the
throttle part for one of these trips - watch your tach as the engine and
transmission work together. The Duratec should be able to hold speed
with the shift lever in OD, with cruise on, easily in any condition that
allows highway speeds unless you are towing... steep hills included. If
your speed falls off 7-10 mph with cruise on, it will disengage. This
would indicate a condition that driving in "D" might be required - like