Hmm, what version of MySQL are you currently running? There are certain versions that are more prone to corruption than others and since the posts table is actually written to less often in this version than prior versions there is definitely something funky going on.
You might try and check the uptime of MySQL to see how long it's actually been running, possibly stopped and restarted during a write? It could be anything like memory, cpu or a hard drive issue.
One thing, make sure you close your forums while trying to repair it. Since it's the post table it can take awhile because it is one of the larger tables. But if your forum is still open while working on it, it can actually corrupt it further.
