1st timer here. I've looked but can't find a specific answer to this. Every now and then when pedaling hard the cranks seem to skip and "clunk" forward. It isn't the chain slipping. I think it is a problem with the ratchet and pall. Can this be repaired or does the entire freewheel mechanism need replacement?
If you're sure it is not the chain slipping then:
First need to figure out if it is a freewheel or cassette. On both, it could be a pall not grabbing correctly. Sometimes you can cure this by flushing the mechanism with some chain lube or light oil. But if that doesn't do it, you'll probably need to replace. For freewheel, that means a whole new freewheel. For cassette, that means a new "freehub body".