We are all having the same problems with our 2004 Nissan Maxima .. This is caused by a defective valve body assembly that Nissan is yet to recall.
However I suggest that everyone experiencing this problem contact Nissan at (800) Nissan 1 immediately.. as I was told the more people that complain, perhaps Nissan will acknowledge the trend and do something about it.
If you just want to fix the problem? You will have to replace your valve body. This can be done by any transmission specialist .. I recommend someone you trust...
The valve body can be removed and reinstalled without having to remove the transmission from the car. The valve body cost $808.00 at your local Nissan dealer, or 3-400 dollars rebuilt, and labor should not be more than 5-600 dollars.. So for a maximum of $1400 .. you WILL fix the slipping problem on your 2004 Nissan Maxima.
I know this is unfair, but rather than come here and vent .. I thought I'd provide you with the information that I took my time finding out to fix my car.. Hope this helps...
Any questions.. feel free to email me at firstname.lastname@example.org