['68 Short Step]

I'm trying to figure out if my torque converter is bad or whats going on with my transmission. When I first crank up the truck and drive around everything works perfect. After I drive it around for a while (20+ miles) the transmission starts to act funny when I accelerate hard or when taking off. feels like the transmission is sightly in neutral or something because the rpm's will go up but the truck is barely moving. After I get up to speed it's ok. Its a th350 trans. Thanks for any advice