2001 Nissan Pathfinder repairs

Hello,
I have a Nissan Pathfinder 2001 (61,000 miles). I have a couple of questions to ask, the first related to oxygen sensors, and the second to transmission repair:

1- my check engine light came on over a year ago, and the dealership replaced one oxygen sensor. Few months later, I took the car to get it checked because I didn't feel it ihas a good pickup on hte highway, they did a full service and replaced two other oxyden sensors. This week, I took it back with the same performance problem, they said two of the oxygen sensors that had been replaced will have to be replaced again. After getting outraged at the fact that they are not giving me the source of the problem, they are now saying that they will clean the fuel injector which could be the source of the oxygen sensors failing and of the driving power. Your thoughts about all this? I am already a $1000 down, without accounting for this last repair!

2- transmission question: a couple of years ago, during a full service, a transmission oil leak was spotted and the transmission pan gasket was replaced. The dealership said that it was still under warranty and fixed the problem. This week, along with the oxygen sensors/fule injector repairs, I am told that there is a transmission oil leak and the transmission pump gasket now needs to be replaced (warranty ran out). The part is super cheap, but when the labor is factored in, that is almost a $1000 repair. Is this oil leak related to the previous one? I am starting to doubt the ability of the mechanics in that shop. Your thoughts?

thanks for your input,
- George