I had some problems this last summer with my tranny. I spoke with DTT transmission and got the fix. I did the service and adjustedthe bands according to the manual. Everthing has been fine. Now that the weather has cooled a little I have noticed on three ocassions a little problem with the tranny. On 2 different times I pulled out of the garage after starting upn from overnight, and after backing out when I put the tranny in drive it revved up but no go. after a few seconds it took off and no further problems. on 1 othe time I had driven for about 10 miles. I stopped for lunch, and after starting out I came to the stop at the end of the drive way and the tranny did not engage immediately. The truck had sat for about 35 minutes. Am I doing something wong here at all other times the tranny is no worries. BTW I do not warm the truck up beyond making sure that the oil pressure is correct. and yes I did check the fluid level.

 

Try letting it idle in neutral for a few minutes.
Check the fluid, also.

 

I think Smokee is on the right track, it sounds like the fluid is draining out of your torque converter. Idling in neutral for a short period of time before moving the truck should cure your problem.

 

switch valve , drain back valve sticking