I'm not the expert on this but I partially agree with craws. I don't know where the check valve is, but I've heard it is somewhere in the cooler return line. That check valve can become held-open with a tiny piece of junk and it allows the torque converter to drain back into the pan. Here's how to confirm this: park it on a level surface for several hours (overnight is better) Before you start it, pull the trans dipstick. Note the fluid level. Reinstall the dipstick. Start it and drive it until it is warm. Park it on a level surface. Leave it running. Pull the trans dipstick. Note the fluid level. If the first check shows a really high level and the second check shows a normal level then you probably have the fluid drainback happening.
The good news is that this isn't really a problem at all. Just start the thing, let it idle for about 15 seconds before shifting into gear and you won't have that delay problem.
By the way, mine has been doing this for the last year and I've had no problems with driveability, shifting, etc...