Originally Posted by dddd

Sounds like a possible ratchet pawl problem where one pawl isn't fully settling into the ratchet ring's teeth until torque is applied.

What kind of freewheel ratchet or cassette body are we talking about here?

It could be in need of lube or it could be a bad pawl spring, but don't spray anything into the hub where you might wash fine grit into clean ball bearings. You need to know the mechanism you're dealing with!