Problem with corrupt index

Hi everybody,
I have a table that requires periodical updates (every 5 seconds) in 
more than one register.
The table is very small (about 7 registers) and it wont grow.
Sometimes the access to that table becomes very slow, and VACUUM dont 
fix it, I have to REINDEX the table.
Anyone can tell me what should be the cause of this problem?

Thanks in advance,

-- 
Diogo de Oliveira Biazus
diogo@ikono.com.br
Ikono Sistemas e Automação
http://www.ikono.com.br