As Sean says, if the lambda sensor will switch lean ( 200mv range ) on decel then it shows that the sensor is capable of sensing oxygen, therefore its just telling you the truth, the mixture is rich. May be worth checking idle manifold vac, if its below 17in/hg it will affect the MAP sensor value in the rich direction.
As a starting point I think it would be helpful if you also posted 4 gas readings at idle then at 2.8k ish. Note what the lambda sensor behaviour is like and the injector durations at these two rpm's. Also note down the fuel trim values. This info will help to decide if the ECU is commanding or fighting the rich mixture. From there we can probably help.