That is why I mentioned on checking the old ATF earlier, as it will provide the answers on what is wrong with the gearbox. When assembling the overhauled gearbox, use fresh ATF out of the can. Never skimp on ATF as this is a vital lubricant inside a dry gearbox, and do not re-use the drained (new) ATF as it is now contaminated with debris from the gearbox internals. Lastly, be sure to drain the torque converter of old ATF.