Our weekly backup failed when trying to backup the postindex table of these forums. This table is used by the search function (which is why that function is currently disabled) and I am currently rebuilding it at the moment. Rebuilding from scratch takes about 18 hours and I'm about 8 hours into it.
While this rebuild occurs, the post table and the postindex table are getting queried very intensively. In addition, whenever someone posts, both tables get written to. This can cause some resource shortages and may in some cases cause the code to abort before completion. Other options include shutting the forums off completely or hacking the code so posts don't get indexed. I'd rather not do either.
The problem you mentioned seems to be intermittent and its symptoms should disappear as soon as the index table is rebuilt.