Sometimes the faces of the bottom bracket lug aren't square. The cups will screw into the threads fine, but then when the lock ring is cranked down, the lug distorts and the bearing balls are not under uniform tension as they roll in their races, causing the clunk when pressed hard as in sprinting and climbing.
One solution is to have the frame faced by a shop that knows what it's doing. The other is to simply buy a cartridge bottom bracket that's compatible with your cranks.