tomecki

Hi, we own Maxima 2000 I guess(it says 8/99 on the door label)
It has 98000 miles on it. I just ran Runrite fuel injection cleaning at Strauss.
Nothing changed. Here is the problem since we bought the car 3 months ago in January.
When I press gas pedal gradually car goes nicely and accelerates adequatly.
But when I press the gas pedal hard, let's say when I start or when for example driving at 30 or 40 miles per hour, then RPM's go to 5 or 6000 but the car stays at the same speed of 40 miles per hour. I had an engine light on, was checked and it showed code p0171 and p 0174 which means bank 1 and bank 2 oxygen sensor. The reader said that the fuel is too lean.
My question is what could be the cause of the problem.
Should I replace oxygen sensors first (if yes, all of them? ( I counted 5 of them))
or it might be transmission or EGR valve or timing or what?
The dealer did not check the car yet but said right away that computer should be upgraded before I decide to change the oxygen sensors for it might not be it.
What do you think. Again, this car goes fast and accelerates fine if the gas pedal is pressed very slowly, but when I want to merge into the traffic and gain the speed rapidly I cannot. Did any of you had similiar problem?
Please let me know. And thanks.
(too me looks like the fuel is supplied because RPM's are up on the gauge, but something halts the speed gain - help !)

Tom