After the truck has been sitting for a while,ex. 4hrs, I start it up, put it in drive and start to go. For the first 5min or so, it will occasionally sit out of gear, almost like itsin neutral, so I have to rev it to get it to lock in (rev, clunk, its in gear). It only does this from a stop or if I let off the gas. I really dont know what's going on, any suggestions?

Thanks!

 

How many miles on the truck, and when was the last time the tranny fluid was changed? Was it just a pan drop or was it a complete exchange? How does the fluid on the stick look, smell, and feel? What kind of fluid was used?

 

It's rare, but the filter may have fallen out of the bore. It happened to me and the symptoms were identical to what you state. Pull the pan and check.