MySQL crashes on a regular basis - once or two times a day (~300,000 PI/day, 10-20 queries per page).
When I repair the tables I _always_ get this message on the 'user' table:

>myisamchk -a user
Checking MyISAM file: user
Data records: 107030 Deleted blocks: 0
myisamchk: warning: Table is marked as crashed
myisamchk: warning: 4 clients is using or hasn't closed the table properly
- check file-size
- check key delete-chain
- check record delete-chain
- check index reference
- check data record references index: 1
- check data record references index: 2
- check data record references index: 3
- check data record references index: 4
- check record links
myisamchk: error: Checksum for key: 4 doesn't match checksum for records
MyISAM-table 'user' is corrupted
Fix it using switch "-r" or "-o"

What's wrong with this index? Why does it always crash?