Nissan Repair: 2002 Nissan Xterra XE Transmission, automatic transmission fluid, nissan xterra

Question
I drive a 2002 Nissan Xterra XE. I have never had any problems with my vehicle in the past, especially my transmission. Lately my transmission has a tough time shifting into overdrive during the morning. I started noticing it this winter.  I am not sure if its the temperature or just the fact that its been sitting all night.  I will let it idle in the morning for a few mintues, get on the road, and when i get to the highway, its shifts through all the other gears fine until it gets to 5th and then it sticks.  It eventually does kick into overdrive after a few minutes of driving on the highway and its not jerky or anything. Very smooth transition but its still concerning.  I have a feeling a valve may be sticking or something. I took it to my mechanic and he says that the transmission fluid is fine and doesn't need to be replaced yet.  So my question is, will a bottle of automatic transmission fluid additives help or is this a bigger problem?

Answer
Patrick,

Change the fluid and filter first.  Second, if it does not do the trick the transmission computer is probably going out.  But, if you run it this summer and all is fine I bet it will start acting up in cold weather again.  That is, if the fluid change did not do the trick.  Do not use addatives on Nissans as they do not like them and it is a last resort before rebuilding.