I keep reading this thread and puzzling over the problem, and keep coming back to something so simple it's probably not what's involved. But I'll throw it out anyway.

When I've had my MTB freehub removed and replaced in the shop, or removed and replace the freewheel on my road bike myself, there's always two or three instances of slippage exactly like what you're describing, as they get tightened up again. The bike mechanic even alerted me to expect that, in the case of the shop repair. There's no way on earth to tighten the things fully other than riding them.

But you didn't say anything about recent removals, so....:confused: