It's because when the fluid is cool it's realy thick and has trouble flowing through some of the smaller orfices and valves. But once it warms up it thins out and flows alot better. That's why you should check the fluid with the truck running and while the tranny is hot.
Depending on how it acts when it acts up is another thing. Lower stall speed is probably normal but shifts funny eh..... What's the gut's of the tranny? If you have rubber seals it might act different than if you were to have some teflon, so who knows unless you know exactly what's in it. It's probably not a problem considering it is fine once warmed up. Look at them like a motor, always let it warm up before beating on it. HTH
