truck changing gears by itself

[mcdonald58]

ever since i did the fluid change on my truck, it sometimes will shift itself into neutral by itself while im stopped at a light or stop sign. it only does it for about the first five minutes of driving then stops. usually by the time im out of my street which is about two minutes it wont happen again. what could be causing this?

 

[mcdonald58]

my transmission is also slipping occasionally. only while the truck warms up though. even if i let it warm up before i drive it the truck will still slip for the first 5 or so minutes of driving. it only slips on first gear.

 

[M-CO]

Is it manual or automatic? Is this the first time you've changed the fluid? What fluid did you use? Are you sure it's filled to the proper level?