As stated earlier you may want to have the cpu codes checked before spending $100-150 on a new O2 sensor. Not that the codes always appear the first time. Back in 1997 I had a similar problem, however mine was only while driving, the rpm's would suddenly drop to idle speed, and a complete loss of power, than all the power would come back. The problem ended up being the speed sensor in the Speedometer. However, the code did not register the first time at the dealership. The O2 sensor could be bad, but the a service rep at the dealer told me that the sensor either works or doesn't, and when it stops sending the signal the cpu expects, the check engine light should come on. The reason I asked is that I have had a 2 mpg loss and a significant increase in hydrocarbons between the last two smog checks on my '90 v6. Suspecting it may be the O2 sensor, as did some of my mechanic friends, I checked the codes myself and nothing had registered.

There could be other sensors going bad such as a speed or idle sensor, a code check may be the cheaper way at the moment.