Oxygen sensor on '95 F150 4X4 5.8EFI?

Since my Truck was new, when I drive over 75 for about 2 hours my "check engine light" comes on. When it was under warranty the dealer said they couldn't find the solution, but adjusted some things and changed the oxygen sensor. It got better for a while but never went away completely, and eventually came back as bad as before.

Some told me it was not a problem and to ignore it.

Others have told me it IS the oxygen sensor. Its been four years now, so should I be changing the O2 sensor anyway or is there another problem?

And...
Is this symptom a sign that I have not been getting optimal performance?

 

Im not sure, but doesn't the Mass air system have 2 o2 sensors? Bosch recommends replacing them at 60k on Fords like ours.

 

only the 96 and up f-150's have more than one o2 sensor

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95 F-150 s/c XLT short 5.0 4x2 4R70W trans. 3.55
Cat-back duels with flo-pro and echo tips
K&N FIPK, Superchip
New dunlop rover touring 255/70R15
Accel 8.8 custom wires, accel superstock coil, accel cap and rotor
Motorcraft platnium plugs
Smoke color bug deflector and vent visors
Better built tool box
Future mods:
Tint job, and a pair of painted oem style cab steps
Front and rear anti-sway bars