2000 Nissan Sentra Oxygen sensors?

Last week my 2000 Sentra's check engine light came on. Almost immediately after the light came on, I noticed that the car was idling funny when at stoplights, stop signs, etc. The car is sputtering almost when idling at a stop, but never actually cuts off. I've also had problems with the car taking off a few times in the last week as well. Sometimes, when I press the gas after I first crank the car, its like the car is not getting any gas and won't move....if I keep pressing the gas pedal then eventually it gets the juice it needs and takes off. We took the car to one car place...they did a tune up and replaced some belts...this did not fix the problem. We took it back and they told us the check engine light was telling them it was the oxygen sensors and that they could repair them for $675. We thought this seemed a little high....so we took the car to another place that said they would charge us less. However, the 2nd place is now telling us that after they did their diagnostic test, that they don't think its the oxygen sensors and can't figure out what is wrong with the car. Any suggestions would be greatly appreciated!!!

Thanks,