It sounds like you're dealing with an internal transmission issue. I am going through some of the same hassles.
My car feels as though it's stuck in, what would be equivalent to 3rd gear in a conventional transmission. The engine will rev but it has a hard time propelling itself from a stand still on anything greater than a flat surface and won't go much past 40mph. For me, it is the step motor in the CVT transmission. It tells the pulleys in the transmission how much to move in order to propel the car. I was at least fortunate enough to have a code to reference.
If you haven't yet gone, your best bet is likely going to be getting a diagnostic from a Nissan dealer. If your local shops are anything like mine, they don't see these transmissions much and don't really know what they're working with.
Unfortunately, you won't get the free warranty replacement for your transmission as that ends at 120k miles. My '03 is sitting at 190k. I got quoted today for $3627.86 to have my local Nissan dealership replace the transmission with a refurbished unit which I will be paying as there is no other alternative since they do not sell parts for them or service them. Looking on the bright side, their trannys come with a 3 year, 36,000 mile warranty so I won't be dealing with this problem again (knock on wood).