[filburt1]

Label me obvious, but things in the computer world never just randomly happen; there's always a problem introduced when the things started happening.

1. Try:
[sql]
SELECT threadid, title FROM thread WHERE visible = 0;
[/sql]
Does it return any significant rows?
2. Get rid of all hacks
3. Get rid of all cron jobs
4. Run the query (although myisamchk should have gotten it anyway):
[sql]
REPAIR TABLE thread;
[/sql]
5. If you're using MySQL 4, downgrade to MySQL 3.