Do you have a service manual on the truck? Somewhere in the torque converter feed/discharge circuit there should be a check valve to keep the converter from draining down fluid. Does the trans jump only after the engine's been shut off or also if the unit's been idling in drive/reverse? Or the converter itself may be faulty-mechanically broken inside. Did you find any metal shavings during the rebuild you couldn't account for? I'd check the stall speed against the spec