I have a problem with my front O2 sensor on my 1991 VR-4.

After the car has warmed up and I drive in fourth or fifth gear at
around 2000 rpm (+/- 100rpm) and the vacuum/boost gauge reads about 5psi
(+/-1 psi), the Check Engine Light will go on.  I need to cruise
constantly for several minutes under the above conditions for the CEL to
turn on.  Checking the code in my manual, it reveals a problem with the
front O2 sensor.  The car has about 91k miles on it.

To characterize this problem, I clear the CEL by disconnecting the
battery for 10 seconds.  Mileage has not decreased from 20-21mpg.

-is this sensor difficult to replace (how many hours by a mechanic, can
I do it myself with simple tools and a sensor wrench)?
-any other comments to help me resolve this?