I've got a 1990 ford ranger, with 2.9 liter EFI package.

About three years ago, I had to replace the O2 sensor after, well, it
'died'.  Symptoms had been loss of power, bucking at highway speeds (on a
flat stretch, having to down shift to maintain speed), and loss of gas
mileage.

    Now I've got a sixty mile commute, and there seems to be one stretch on
the way up in the after noon (I work nights) where I seem to have to shift
down (fifth to fourth) to 'clear' the bucking, and then I can shift back up
to 5th.
    So I'm suspecting the O2 sensor (again) but does anyone have any idea
how long those are suppose to last.  Its been about 3 years and 50,00 miles
if that is any help.

thanks in advance for any help.

peter