I have a 2000 F-150 Lariat 5.4, in the morning when I first start the truck it goes in reverse with no problem, once I drive it for a little while it won't go in reverse until it seats for a few hours, any ideas what the problem is? I did flush the transmission fluid and change the filter.

 

Most likely it is worn seals in the reverse clutch. The seals work when cold, but when the fluid thins as it warms the seals leak. That's my guess.