transmission shifts while on the highway on 2000 Nissan Maxima

I have 65k miles on my car. Whenever i drive on the highway
and the engine is still cold, it takes about 4-5 miles or
20 min of driving for the car to shift into overdrive.
When the car was new the car shift into overdrive right away,
but it the last 2 yrs, it takes longer to shift into OD.
I read online that it could be some type of solenoid.
Is this a transmission problem or just replace the solenoid?
thanks

1 answer 1 comment
dont think it is the soloniod. it may be a sticking valve in the valve body. when was the last time you had the fluid serviced? thats a good start with a valve body modifier added as well.

Roy
the fliud was last changed in 2006.