Transmission flairing or slipping
 

I have a 1980 300D with a 722.1 trans. The transmission seems to shift properly at low throttle-torque conditions. But seems to flair or slip under high throttle-torque conditions when shifting. The trans. has been rebuilt, all except the valve body. It has a new modulator valve and i think proper vacuum. I'm thinking it's in the valve body.
Can anyone point me in the right direction. What should I do next? THANKS FOR LOOKING. ANY HELP APPRECIATED!